Ma has an afterthought - a complaint (5)
I believe the answer is:
mumps
'a complaint' is the definition.
(mumps is a kind of health problem)
'ma has an afterthought' is the wordplay.
'ma' becomes 'mum' (synonyms).
'has' says to put letters next to each other.
'an afterthought' becomes 'ps' (ps stands for postscript).
'mum'+'ps'='MUMPS'
(Other definitions for mumps that I've seen before include "Epidemic parotitis" , "Infectious childhood disease" , "Childhood infection" , "Viral disease with swellings" , "Contagious viral disease" .)