A mother has a right to spoil! (3)
I believe the answer is:
mar
'spoil' is the definition.
(synonyms)
'a mother has a right' is the wordplay.
'a mother' becomes 'ma' (ma can mean mother).
'has' means one lot of letters go next to another.
'a right' becomes 'r' (common abbreviation).
'ma'+'r'='MAR'
'to' is the link.
(Other definitions for mar that I've seen before include "Oh dear!" , "A windy month" , "when spring growth starts?" , "Ram into and spoil" , "Make imperfect" .)