Feeble joke — start to yawn (4)
I believe the answer is:
puny
'feeble' is the definition.
(I've seen this in another clue)
'joke start to yawn' is the wordplay.
'joke' becomes 'pun' (pun is a type of joke).
'start to' indicates taking the first letters.
The initial letter of 'yawn' is 'y'.
'pun'+'y'='PUNY'
(Other definitions for puny that I've seen before include "in need of development" , "Weak - unimportant" , "Slight" , "Insignificant" , "Small and weak" .)