Loudly complain - that's feeble (5)
I believe the answer is:
frail
'feeble' is the definition.
(similar in meaning)
'loudly complain' is the wordplay.
'loudly' becomes 'f' (short for forte - 'loud' in music).
'complain' becomes 'rail' (railing is a kind of complaining).
'f'+'rail'='FRAIL'
'that's' acts as a link.
(Other definitions for frail that I've seen before include "old and weak" , "Physically weak, delicate" , "isn't sturdy" , "Delicate, fragile" , "hobbling around?" .)