Active American serviceman having beer outside (5)
I believe the answer is:
agile
'active' is the definition.
(similar in meaning)
'american serviceman having beer outside' is the wordplay.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'serviceman' becomes 'gi' (informal term for a US soldier).
'having beer outside' becomes 'le' (I can't explain this - if you can you should believe this answer much more).
'a'+'gi'+'le'='AGILE'
Can you help me to learn more?
(Other definitions for agile that I've seen before include "Nimble, supple" , "Quick and easy-moving" , "Nifty" , "Nimble, fleet" , "Able to move nimbly" .)