Spoil a letter to mum (3)
I believe the answer is:
mar
'spoil' is the definition.
(synonyms)
'a letter to mum' is the wordplay.
'a letter' becomes 'r' ().
'to' is a charade indicator (letters next to each other) (I've seen this in other clues).
'mum' becomes 'ma' (both can mean mother).
'r' put after 'ma' is 'MAR'.
(Other definitions for mar that I've seen before include "Spoil, damage" , "Spoil or deface" , "Ruin" , "Cause harm" , "Spoil something" .)