Mother has second thoughts after contagious disease (5)
I believe the answer is:
mumps
'contagious disease' is the definition.
(I've seen this before)
'mother has second thoughts' is the wordplay.
'mother' becomes 'mum' (informal term for mother).
'has' is a charade indicator (letters next to each other).
'second thoughts' becomes 'ps' (as in P.S. in a letter).
'mum'+'ps'='MUMPS'
'after' acts as a link.
(Other definitions for mumps that I've seen before include "Disease once common in children" , "Epidemic parotitis" , "Parotid gland disease" , "Infectious disease, making neck glands swell" , "Viral disease mainly affecting children" .)