Feeble witticism by Yankee (4)
I believe the answer is:
puny
'feeble' is the definition.
(I've seen this before)
'witticism by yankee' is the wordplay.
'witticism' becomes 'pun' (pun is a kind of witticism).
'by' says to put letters next to each other.
'yankee' becomes 'Y' (phonetic alphabet: alpha, bravo, charlie etc.).
'pun'+'y'='PUNY'
(Other definitions for puny that I've seen before include "Small and weak" , "Slight" , "Paltry" , "Insignificant" , "Feeble, under-sized" .)