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' means one lot of letters go next to another.
'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" .)

I've seen this clue in the Evening Standard.

Want a hint initially instead of a full solution? Install my app