Hit on the head with old coin (5)
I believe the answer is:
crown
I believe this is a double definition.
'hit on the head' is the first definition.
(I've seen this before)
'old coin' is the second definition.
(I've seen this before)
'with' is the link.
(Other definitions for crown that I've seen before include "piece of tooth" , "Old coin; whack on head" , "Hit on the head (informal)" , "Symbol of royalty" , "The sovereign" .)