Nimble US soldier drowning in beer (5)
I believe the answer is:
agile
'nimble' is the definition.
(agile can mean nimble or athletic)
'us soldier drowning in beer' is the wordplay.
'us soldier' becomes 'GI' (GI can mean an American soldier).
'drowning in' means one lot of letters goes inside another.
'beer' becomes 'ale' (type of beer).
'gi' inserted into 'ale' is 'AGILE'.
(Other definitions for agile that I've seen before include "Sprightly" , "Quick and easy-moving" , "Mentally acute" , "Nimble, quick-moving" , "Flexible" .)