This doesn't do exactly what you want, but it's pretty close.
If you type /cl and /clear appears in the editbox, you'll have to expand your search term to get anything else, so say you wanted to get /close you would type /clo instead of /cl.
You use F2 instead of the tab key.

Code:
alias F2 {
  hadd -m autocomplete 1 abook ajinvite alias aline ame amsg anick aop auser avoice away background ban bcopy beep bread break breplace bset btrunc $&
    bunset bwrite channel clear clearall cline clipboard close cnick color colour comclose comopen comreg continue copy creq ctcpreply ctcps $&
    dcc dccserver dde ddeserver debug dec describe did didtok disable disconnect dlevel dline dll dns dqwindow drawcopy drawdot drawfill
  hadd -m autocomplete 2 drawline drawpic drawrect drawreplace drawrot drawsave drawscroll drawtext ebeeps echo editbox emailaddr enable events exit $&
    fclose filter findtext finger firewall flash flist flood flush flushini font fopen fseek fsend fserve fullname fwrite ghide gload gmove $&
    gopts goto gplay gpoint gqreq groups gshow gsize gstop gtalk gunload guser hadd halt haltdef hdec hdel help hfree hinc hload hmake hop
  hadd -m autocomplete 3 hsave ial ialclear ialmark identd if ignore iline inc invite iuser join kick linesep links list load loadbuf localinfo log $&
    mdi me mkdir mode msg nick notice notify omsg onotice part partall pdcc perform play playctrl pop protect pvoice qme qmsg query queryrn $&
    quit raw reload remini remote remove rename renwin reseterror resetidle return rlevel rline rmdir run ruser save savebuf saveini say scid
  hadd -m autocomplete 4 scon server set showmirc signal sline sockaccept sockclose socklist socklisten sockmark sockopen sockread sockrename sockudp $&
    sockwrite sound speak splay sreq strip timer timestamp titlebar tnick tokenize topic tray ulist unload unset unsetall updatenl url uwho $&
    var vcadd vcmd vcrem vol while whois window winhelp write writeini
  var %i = 1, %s, %s2 = $regsub($editbox($active),/\//g,,%s), %s3 = $count($editbox($active),/), %r2
  while ($hget(autocomplete,%i)) {
    if ($wildtok($hget(autocomplete,%i),$+(%s,*),1,32)) { 
      %r2 = $v1 
      .break
    }
    inc %i
  }
  if (!%r2) {
    var %i = 1, %i2, %r, %r2, %s, %s2 = $regsub($editbox($active),/\//g,,%s), %s3 = $count($editbox($active),/)
    while ($alias(%i)) {
      if ($fopen(autocomplete)) { .fclose autocomplete }
      .fopen autocomplete $+(",$alias(%i).fname,")
      while (!$feof) && (!%r2) {
        %r = $fread(autocomplete)
        if ($regex(%r,/^(?:n\d+=)( $+ %s $+ \S+) .+$/)) && (!%i2) { 
          %r2 = $regml(1)
          .break
        }
        inc %i2 $count(%r,{)
        dec %i2 $count(%r,})
      }
      if ($fopen(autocomplete)) { .fclose autocomplete }
      inc %i
    }
    if (!%r2) {
      var %i = 1, %i2, %r, %r2, %s, %s2 = $regsub($editbox($active),/\//g,,%s), %s3 = $count($editbox($active),/)
      while ($script(%i)) {
        if ($fopen(autocomplete)) { .fclose autocomplete }
        .fopen autocomplete $+(",$script(%i).fname,")
        while (!$feof) && (!%r2) {
          %r = $fread(autocomplete)
          if ($regex(%r,/^(?:n\d+=)?alias ( $+ %s $+ \S+) .+$/)) && (!%i2) { 
            %r2 = $regml(1)
            .break
          }
          inc %i2 $count(%r,{)
          dec %i2 $count(%r,})
        }
        if ($fopen(autocomplete)) { .fclose autocomplete }
        inc %i
      }
    }
  }
  editbox -af $iif(%r2,$+($str(/,%s3),%r2),$editbox($active))
  .hfree autocomplete
}


The code's messy because I made it pretty quickly, i'll see if I can clear it up and i'll post the new one later.