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' says to take the initial letters.
The first 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" .)