Drowsy agent outside shelter (6)
I believe the answer is:
sleepy
'drowsy' 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' enclosing 'lee' is 'SLEEPY'.
(Other definitions for sleepy that I've seen before include "over-tired" , "Dozy" , "in retirement?" , "Rather tired" , "Feeling tired" .)