Many a game spoilt by injury (6)
I believe the answer is:
damage
'injury' is the definition.
(I know that damage is a type of injury)
'many a game spoilt' is the wordplay.
'many' becomes 'D' (Roman numeral for 500).
'spoilt' indicates anagramming the letters.
'game' anagrammed gives 'mage'.
'd'+'a'+'mage'='DAMAGE'
'by' is the link.
(Other definitions for damage that I've seen before include "To vandalise" , "Impair, hurt" , "destruction" , "Deface, harm" , "Cost (slang)" .)