Silent afterthought in complaint (5)
I believe the answer is:
mumps
'complaint' is the definition.
(mumps is a kind of health problem)
'silent afterthought' is the wordplay.
'silent' becomes 'mum' (synonyms).
'afterthought' becomes 'ps' (ps stands for postscript).
'mum'+'ps'='MUMPS'
'in' is the link.
(Other definitions for mumps that I've seen before include "Childhood infection" , "Complaint" , "Virus disease causing painful swellings in neck" , "Viral disease, dangerous to adult males" , "Swelling disease" .)