Spoil a short month (3)
I believe the answer is:
mar
I believe this is a double definition.
'spoil' is the first definition.
(I know that spoil can be written as mar)
'a short month' is the second definition.
(short for March; 'short' indicates a short form)
(Other definitions for mar that I've seen before include "Ruin" , "Blemish" , "Cause harm" , "Spoil something" , "Oh dear!" .)