Tired agent outside shelter (6)
I believe the answer is:
sleepy
'tired' is the definition.
(thesaurus)
'agent outside shelter' is the wordplay.
'agent' becomes 'spy' (spy is a kind of agent).
'outside' means one lot of letters goes inside another.
'shelter' becomes 'lee' (I've seen this before).
'spy' placed around 'lee' is 'SLEEPY'.
(Other definitions for sleepy that I've seen before include "Drowsy - without excitement" , "Somnolent" , "Dwarf" , "Ready to turn in" , "(Of place) low in activity" .)