if ($(a &) iswm $1-) { }
iswm is the wildcard operator, '&' is a wildcard matching a word, $() is required because '&' itself can be an operator and mIRC would think you are doing if (a & iswm $1-)
Of course, typically, we put this as the match text of the event:
on *:text:a &:#:{
and this will only match when the message is of the format "a <word>".
You probably don't need $rawmsg, use $1- to refer to the full message.
