Adult's heard making painful sound (5)
I believe the answer is:
groan
'making painful sound' is the definition.
'adult's heard' is the wordplay.
'adult' becomes 'grown' ('grown' can be a synonym of 'adult').
'heard' shows a homophone (sound like).
'grown' is a homophone of 'GROAN'.
(Other definitions for groan that I've seen before include "painful sigh" , "express disapproval" , "Sound pained; Gormenghast earldom" , "Cry of pain or despair" , "Titus -- (Mervyn Peake)" .)