Topless girl friend (4)
I believe the answer is:
ally
'friend' is the definition.
(an ally is a friend or supporter)
'topless girl' is the wordplay.
'topless' means to remove the first letter.
'girl' becomes 'sally' (feminine given name).
'sally' with its initial letter removed is 'ALLY'.
(Other definitions for ally that I've seen before include "Accomplice" , "often helpful when cluing adverbs" , "Comrade in arms" , "Co-operative friend" , "mate" .)