Mad gene used in final stage (7)
I believe the answer is:
endgame
'final stage' is the definition.
Both the definition and answer are singular nouns.
Perhaps you can see an association between them that I don't see?
'mad gene used' is the wordplay.
'used' is an anagram indicator.
'mad'+'gene'='madgene'
'madgene' with letters rearranged gives 'ENDGAME'.
'in' acts as a link.
Can you help me to learn more?
(Other definitions for endgame that I've seen before include "Closing chess moves" , "close to board meeting" , "Final stage in a contest" , "closing moves on chessboard" , "Beckett play" .)