One who plays it badly may finish out of pocket (9)
I believe the answer is:
billiards
'one who plays it' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'badly may finish out of pocket' is the wordplay.
I cannot really see how this works, but
'badly' could be 'ill' ('ill' can be adverbial as in 'ill-advised') and 'ill' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for billiards that I've seen before include "Table game at which Mary Queen of Scots excelled" , "Game played with balls on table with pockets" , "Ball game" , "Indoor sport" , "Game with three balls" .)