A soldier on lake with energy, nimble (5)
I believe the answer is:
agile
'nimble' is the definition.
('agile' can be a synonym of 'nimble')
'a soldier on lake with energy' is the wordplay.
'soldier' becomes 'gi' (government issue or general issue**).
'on' means one lot of letters go next to another.
'lake' becomes 'l'.
'with' says to put letters next to each other.
'energy' becomes 'e' (physics symbol as in E=mci).
'a'+'gi'+'l'+'e'='AGILE'
(Other definitions for agile that I've seen before include "Mentally acute" , "athletic" , "Quick in movement" , "Nimble on the feet" , "Nimble, active" .)