Boy imprisoned by those people, say, makes regular complaint (5,4)
I believe the answer is:
theme song
'regular complaint' is the definition.
(can mean a recurrent topic or expression)
'boy imprisoned by those people say' is the wordplay.
'boy' becomes 'son' ('son' can be a synonym of 'boy').
'imprisoned by' is an insertion indicator.
'those people' becomes 'them' (I've seen this before).
'say' becomes 'eg'.
'them'+'eg'='themeg'
'son' put into 'themeg' is 'THEME SONG'.
'makes' is the link.
(Other definitions for theme song that I've seen before include "Signature tune" , "Characteristic melody often repeated in musical" , "Give me Sunshine" .)