Spoil master Rex? (3)
I believe the answer is:
mar
'spoil' is the definition.
(I know that spoil can be written as mar)
'master rex?' is the wordplay.
'master' becomes 'ma' (Master of Arts).
'rex?' becomes 'R' (abbreviation e.g. GR=George Rex, King George).
'ma'+'r'='MAR'
(Other definitions for mar that I've seen before include "Spoil something" , "Damage; month (abbr.)" , "when spring growth starts?" , "Disfigure" , "Spanish main?" .)