Spoil master Rex? (3)
I believe the answer is:
mar
'spoil' is the definition.
(synonyms)
'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?" .)