Backsliding agent pockets bung (7)
I believe the answer is:
stopper
'bung' is the definition.
(I know that bung can be written as stopper)
'backsliding agent pockets' is the wordplay.
'backsliding' shows that the letters should be reversed in order.
'agent' becomes 'rep' (short for representative).
'pockets' becomes 'pots' (to pocket or pot a ball in snooker).
'rep'+'pots'='reppots'
'reppots' back-to-front is 'STOPPER'.
(Other definitions for stopper that I've seen before include "Plug for a bottle" , "Cork - goalkeeper?" , "Bung" , "One blocks" , "Plug - cork" .)