By the sound of it, poem due (4)
I believe the answer is:
owed
'due' is the definition.
('owed' can be a synonym of 'due')
'by the sound of it poem' is the wordplay.
'by the sound of it' indicates a 'sounds like' (homophone) clue.
'poem' becomes 'ode' (ode is a kind of poem).
'ode' is a homophone of 'OWED'.
(Other definitions for owed that I've seen before include "(Sum) due for payment" , "to be paid" , "Was indebted" , "Was due to pay" , "Was obliged to pay" .)