Hiding head, fool friend (4)
I believe the answer is:
ally
'friend' is the definition.
(synonyms)
'hiding head fool' is the wordplay.
'hiding head' means to remove the first letter.
'fool' becomes 'wally' (I've seen this before).
'wally' with its first letter taken off is 'ALLY'.
(Other definitions for ally that I've seen before include "Associate that provides assistance" , "Cooperating country" , "Join - friend" , "Crony" , "couple?" .)