Exactly where the hammer should land (2,3,4)
I believe the answer is:
on the nail
'exactly' is the definition.
(I've seen this before)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for on the nail that I've seen before include "Where varnish is put" , "Where a coat may be hung" , "immediately" , "location of solution for digital protection" , "without delay" .)