Spoil a letter to mother (3)
I believe the answer is:
mar
'spoil' is the definition.
(synonyms)
'a letter to mother' is the wordplay.
'a letter' becomes 'r' ().
'to' means one lot of letters go next to another (I've seen this in other clues).
'mother' becomes 'ma' (ma can mean mother).
'r' after 'ma' is 'MAR'.
(Other definitions for mar that I've seen before include "wreck" , "when spring growth starts?" , "Spoil, damage" , "Blight" , "Detract from" .)