Hit on the head (4)
I believe the answer is:
conk
'hit on the head' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for conk that I've seen before include "Hooter" , "Nose, head (sl.)" , "(Slang) nose - head" , "Nose (colloq.)" , "Nose; break down" .)