Active serviceman's parting drink (5)
I believe the answer is:
agile
'active' is the definition.
(similar in meaning)
'serviceman's parting drink' is the wordplay.
'serviceman' becomes 'gi' (informal term for a US soldier).
'parting' indicates putting letters inside.
'drink' becomes 'ale' (type of beer).
'gi' going into 'ale' is 'AGILE'.
(Other definitions for agile that I've seen before include "Limber" , "Moving quickly/easily" , "Quick in movement" , "Mentally acute" , "Nimble, lithe" .)