Angry inmate harbours large complaint (7)
I believe the answer is:
ailment
'complaint' is the definition.
(I know that complaint can be written as ailment)
'angry inmate harbours large' is the wordplay.
'angry' indicates anagramming the letters.
'harbours' is an insertion indicator.
'large' becomes 'L' (eg in clothes sizes).
'inmate' anagrammed gives 'aiment'.
'aiment' going around 'l' is 'AILMENT'.
(Other definitions for ailment that I've seen before include "Slight but persistent illness" , "Minor illness" , "Malady" , "Tummy upset, for instance" , "I lament (anag.)" .)