Strikes one an instant after, it's funny (9)
I believe the answer is:
slapstick
'funny' is the definition.
(thesaurus)
'strikes one an instant after' is the wordplay.
'strikes' becomes 'slaps' (slapping is a kind of striking**).
'one an instant' becomes 'tick' (I've seen this before).
'after' means one lot of letters go next to another.
'slaps'+'tick'='SLAPSTICK'
'it's' is the link.
(Other definitions for slapstick that I've seen before include "Boisterous comedy based on actions rather than words" , "Crude form of comedy" , "That's laughable" , "Physical form of comedy" , "Simple comic act" .)