Make money in poorly performing sport (9)
I believe the answer is:
badminton
'sport' is the definition.
(badminton is a kind of sport)
'make money in poorly performing' is the wordplay.
'make money' becomes 'mint' (as in minting new coins).
'in' means one lot of letters goes inside another.
'poorly' becomes 'bad' (both can mean unwell).
'performing' becomes 'on' (as in being on stage).
'bad'+'on'='badon'
'mint' placed inside 'badon' is 'BADMINTON'.
(Other definitions for badminton that I've seen before include "Seat of the Duke of Beaufort - racket game" , "Racquet sport" , "Racket game with shuttlecock" , "Game played with rackets" , "One sport" .)