Idea of one game gone wrong (5)
I believe the answer is:
image
'idea' is the definition.
(I've seen this in another clue)
'one game gone wrong' is the wordplay.
'one' becomes 'i' (Roman numeral).
'gone wrong' indicates an anagram.
'game' is an anagram of 'mage'.
'i'+'mage'='IMAGE'
'of' acts as a link.
(Other definitions for image that I've seen before include "Pictorial representation" , "Mental picture; metaphor" , "personality" , "Simulacrum" , "Visible representation" .)