Fool's topless friend (4)
I believe the answer is:
ally
'friend' is the definition.
(an ally is a friend or supporter)
'fool's topless' is the wordplay.
'fool' becomes 'wally' (wally can mean a stupid person).
'topless' means to remove the first letter.
'wally' with its initial letter taken away is 'ALLY'.
(Other definitions for ally that I've seen before include "Join - friend" , "Colleague, confederate" , "One on one's side" , "One in cooperation with another" , "often helpful when cluing adverbs" .)