Agent accepts shelter when tired (6)
I believe the answer is:
sleepy
'tired' is the definition.
(thesaurus)
'agent accepts shelter' is the wordplay.
'agent' becomes 'spy' (spy is a kind of agent).
'accepts' indicates putting letters inside.
'shelter' becomes 'lee' (I've seen this before).
'spy' going around 'lee' is 'SLEEPY'.
'when' acts as a link.
(Other definitions for sleepy that I've seen before include "Rather tired" , "Ready to turn in" , "Lethargic" , "Dwarf" , "Unwakeful" .)