Active US soldier with bitter exterior (5)
I believe the answer is:
agile
'active' is the definition.
(similar in meaning)
'us soldier with bitter exterior' is the wordplay.
'us soldier' becomes 'GI' (informal term for a US soldier).
'with bitter' becomes 'ale' (type of beer or bitter. I am not sure about the 'with' bit.).
'exterior' is an insertion indicator.
'gi' put into 'ale' is 'AGILE'.
Can you help me to learn more?
(Other definitions for agile that I've seen before include "Lithely moving" , "Quick and easy-moving" , "Nimble, active" , "Quick-moving, supple" , "Nimble, lively" .)