Small play on words ending in hilarity (4)
I believe the answer is:
puny
'small' is the definition.
(I've seen this before)
'play on words ending in hilarity' is the wordplay.
'play on words' becomes 'pun' ('pun' can be a synonym of 'play on words').
'ending in' indicates one should take the final letters.
The last letter of 'hilarity' is 'y'.
'pun'+'y'='PUNY'
(Other definitions for puny that I've seen before include "Weak - unimportant" , "Paltry" , "Feeble, under-sized" , "Small and feeble" , "Slight" .)