Spoil part of the Common Market (3)
I believe the answer is:
mar
'spoil' is the definition.
(synonyms)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for mar that I've seen before include "Blight" , "wreck" , "Spanish main?" , "Disfigure" , "Damage; month (abbr.)" .)