Mother has an afterthought - a complaint (5)
I believe the answer is:
mumps
'a complaint' is the definition.
(mumps is a kind of health problem)
'mother has an afterthought' is the wordplay.
'mother' becomes 'mum' (informal term for mother).
'has' is a charade indicator (letters next to each other).
'an afterthought' becomes 'ps' (as in a PS in a letter).
'mum'+'ps'='MUMPS'
(Other definitions for mumps that I've seen before include "Infectious glandular disease" , "Complaint" , "Infectious disease, making neck glands swell" , "Viral disease with swellings" , "Viral disease, dangerous to adult males" .)