Parent has final word in complaint (5)
I believe the answer is:
mumps
'complaint' is the definition.
(mumps is a kind of disease)
'parent has final word' is the wordplay.
'parent' becomes 'mum' (mum is a kind of parent**).
'has' is a charade indicator (letters next to each other).
'final word' becomes 'ps' (PS in a letter).
'mum'+'ps'='MUMPS'
'in' is the link.
(Other definitions for mumps that I've seen before include "Epidemic parotitis" , "Disease once common in children" , "Infectious glandular disease" , "Virus disease causing painful swellings in neck" , "Viral disease, dangerous to adult males" .)