To finish up as head (3)
I believe the answer is:
top
'head' is the definition.
(synonyms)
'to finish up' is the wordplay.
'finish' suggests halving the letters.
'up' cut in half is 'p'.
'to'+'p'='TOP'
'as' is the link.
(Other definitions for top that I've seen before include "Toy; uppermost point" , "Peak; child's toy" , "premier" , "Zenith" , "Summit or spinning toy" .)