Weak play on words, ultimately ropy (4)
I believe the answer is:
puny
'weak' is the definition.
('puny' can be similar in meaning to 'weak')
'play on words ultimately ropy' is the wordplay.
'play on words' becomes 'pun' ('pun' can be a synonym of 'play on words').
'ultimately' says to take the final letters.
The final letter of 'ropy' is 'y'.
'pun'+'y'='PUNY'
(Other definitions for puny that I've seen before include "Slight" , "Frail and weedy" , "Insignificant" , "Feeble, under-sized" , "Weak - unimportant" .)