Killing jokes' end with hilarity (9)
I believe the answer is:
slaughter
'killing' is the definition.
(slaughter is a kind of killing)
'jokes' end with hilarity' is the wordplay.
'end' indicates one should take the final letters.
'with' means one lot of letters go next to another.
'hilarity' becomes 'laughter' (I've seen this before).
The last letter of 'jokes' is 's'.
's'+'laughter'='SLAUGHTER'
(Other definitions for slaughter that I've seen before include "Glare thus to cause bloodthirsty killing" , "in which many die" , "Widespread killing" , "Mass killing or carnage" , "Wholesale killing" .)