Inequitable ruin of fun by tune (6)
I believe the answer is:
unfair
'inequitable' is the definition.
(I've seen this before)
'ruin of fun by tune' is the wordplay.
'ruin of' indicates an anagram.
'by' means one lot of letters go next to another.
'tune' becomes 'air' (air can mean a piece of music).
'fun' is an anagram of 'unf'.
'unf'+'air'='UNFAIR'
(Other definitions for unfair that I've seen before include "Unjust, partial" , "Not equitable, biased" , "Not equitable - or blonde?" , "Not equitable-or blonde?" , "Not equitable or blond" .)