Of a pair, the one spoiling most of the game with an own goal (5)
I believe the answer is:
magog
'of a pair' is the definition.
The definition suggests a singular noun which matches the answer.
'the one spoiling most of the game with an own goal' is the wordplay.
'the one spoiling' is an anagram indicator.
'most of' means to remove the last letter.
'with' says to put letters next to each other.
'an own goal' becomes 'og' (football abbreviation).
'game' with its last letter taken off is 'gam'.
'gam' anagrammed gives 'mag'.
'mag'+'og'='MAGOG'
Can you help me to learn more?
(Other definitions for magog that I've seen before include "Biblical king" , "Gog's companion" , "Gog and -, City of London guardian giants" , "Last of two old giants" , "old giant" .)