Feeble joke, ultimately unfunny (4)
I believe the answer is:
puny
'feeble' is the definition.
(thesaurus)
'joke ultimately unfunny' is the wordplay.
'joke' becomes 'pun' (punning is a kind of joking).
'ultimately' indicates one should take the final letters.
The last letter of 'unfunny' is 'y'.
'pun'+'y'='PUNY'
(Other definitions for puny that I've seen before include "Frail and weedy" , "Feeble, under-sized" , "Small and feeble" , "Slight" , "Paltry" .)