Mum has the right to spoil! (3)
I believe the answer is:
mar
'spoil' is the definition.
(I know that spoil can be written as mar)
'mum has the right' is the wordplay.
'mum' becomes 'ma' (ma can mean mother).
'has' says to put letters next to each other.
'the right' becomes 'r' (common abbreviation).
'ma'+'r'='MAR'
'to' is the link.
(Other definitions for mar that I've seen before include "Cause harm" , "Make imperfect" , "Spoil, damage" , "Spoil something" , "Ruin" .)