Agent keeping general quiet (6)
I believe the answer is:
sleepy
'quiet' is the definition.
(thesaurus)
'agent keeping general' is the wordplay.
'agent' becomes 'spy' (spy is a kind of agent).
'keeping' means one lot of letters goes inside another.
'general' becomes 'lee' (Robert E. Lee - US revolutionary commander).
'spy' placed around 'lee' is 'SLEEPY'.
(Other definitions for sleepy that I've seen before include "Dwarf" , "Dozy" , "over-tired" , "Feeling tired" , "Unwakeful" .)