A complaint when mother’s about to finish up (5)
I believe the answer is:
mumps
'a complaint' is the definition.
(mumps is a kind of disease)
'mother's about to finish up' is the wordplay.
'mother's' becomes 'mums' (mum is a kind of mother**).
'about' is an insertion indicator.
'to finish' indicates one should take the final letters.
The last letter of 'up' is 'p'.
'mums' enclosing 'p' is 'MUMPS'.
'when' is the link.
(Other definitions for mumps that I've seen before include "harmful disease" , "Viral disease mainly affecting children" , "Acute viral disease, often in children" , "Swollen-face disease" , "Epidemic parotitis" .)