Agent admits the general is lethargic (6)
I believe the answer is:
sleepy
'lethargic' is the definition.
(I've seen this in another clue)
'agent admits the general' is the wordplay.
'agent' becomes 'spy' (spy is a kind of agent).
'admits' means one lot of letters goes inside another (to admit is to allow in).
'the general' becomes 'Lee' (Robert E. Lee - US revolutionary commander).
'spy' placed around 'lee' is 'SLEEPY'.
'is' acts as a link.
(Other definitions for sleepy that I've seen before include "over-tired" , "One of the Seven Dwarfs" , "Dozy" , "(Of place) without much activity" , "Somnolent" .)