Rover knocking back pint, perhaps, during game, informally (10)
I believe the answer is:
freebooter
'rover' is the definition.
(both can mean an adventurer)
'knocking back pint perhaps during game informally' is the wordplay.
'knocking back' says the letters should be written in reverse.
'pint perhaps' becomes 'beer' (pint of beer).
'during' means one lot of letters goes inside another.
'game informally' becomes 'footer' (informal term for football).
'beer' in reverse letter order is 'reeb'.
'reeb' placed inside 'footer' is 'FREEBOOTER'.
(Other definitions for freebooter that I've seen before include "Person roaming for plunder" , "Rover" , "Lawless adventurer" , "Pirate" , "Someone living off plunder" .)