Active serviceman seen in drink (5)
I believe the answer is:
agile
'active' is the definition.
(similar in meaning)
'serviceman seen in drink' is the wordplay.
'serviceman' becomes 'gi' (informal term for a US soldier).
'seen in' means one lot of letters goes inside another.
'drink' becomes 'ale' (type of beer).
'gi' inserted inside 'ale' is 'AGILE'.
(Other definitions for agile that I've seen before include "Lithely moving" , "alert" , "Spry - energetic" , "Quick in movement" , "Nifty" .)