Spoil a letter to mum (3)
I believe the answer is:
mar
'spoil' is the definition.
('mar' can be a synonym of 'spoil')
'a letter to mum' is the wordplay.
'a letter' becomes 'r' (**).
'to' means one lot of letters go next to another (I've seen this in other clues).
'mum' becomes 'ma' ('ma' can be a synonym of 'mum').
'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" .)