Marry count after losing head (4)
I believe the answer is:
ally
'marry' is the definition.
(thesaurus)
'count after losing head' is the wordplay.
'count' becomes 'tally' (synonyms).
'after losing head' means to remove the first letter.
'tally' with its initial letter removed is 'ALLY'.
(Other definitions for ally that I've seen before include "Crony" , "Join - friend" , "couple?" , "Band together" , "Accomplice" .)