Knock it on the head, initially! (3)
I believe the answer is:
hit
'knock' is the definition.
(synonyms)
'it on the head initially' is the wordplay.
'on' is a charade indicator (letters next to each other).
'initially' indicates taking the first letters.
The initial letter of 'head' is 'h'.
'it' after 'h' is 'HIT'.
(Other definitions for hit that I've seen before include "Thump" , "suit" , "Successful record" , "Popular song; strike" , "Best-selling record" .)