Fool with friend (5)
I believe the answer is:
wally
'fool' is the definition.
(wally can mean a stupid person)
'with friend' is the wordplay.
'with' becomes 'w' (abbreviation).
'friend' becomes 'ally' (an ally is a friend or supporter).
'w'+'ally'='WALLY'
(Other definitions for wally that I've seen before include "A fool (sl.)" , "Idiot (slang)" , "Complete idiot" .)