Joke ultimately silly and feeble (4)
I believe the answer is:
puny
'feeble' is the definition.
(thesaurus)
'joke ultimately silly' is the wordplay.
'joke' becomes 'pun' (punning is a kind of joking).
'ultimately' suggests the final letters.
The last letter of 'silly' is 'y'.
'pun'+'y'='PUNY'
'and' acts as a link.
(Other definitions for puny that I've seen before include "Paltry" , "in need of development" , "Small and weak" , "Insignificant" , "Feeble, under-sized" .)