Swollen, we hear, producing bellyache (5)
I believe the answer is:
groan
'bellyache' is the definition.
(thesaurus)
'swollen we hear' is the wordplay.
'swollen' becomes 'grown'.
'we hear' indicates a 'sounds like' (homophone) clue.
'grown' is a homophone of 'GROAN'.
'producing' is the link.
(Other definitions for groan that I've seen before include "painful sigh" , "Titus -- (Mervyn Peake)" , "Make pained sound" , "Sound of pain or disapproval" , "Cry of pain or despair" .)