Spoil mother rotten, initially (3)
I believe the answer is:
mar
'spoil' is the definition.
(I know that spoil can be written as mar)
'mother rotten initially' is the wordplay.
'mother' becomes 'ma' (ma can mean mother).
'initially' says to take the initial letters.
The first letter of 'rotten' is 'r'.
'ma'+'r'='MAR'
(Other definitions for mar that I've seen before include "Spoil, damage" , "Ruin" , "Damage; month (abbr.)" , "A windy month" , "Cause harm" .)