Agent accepts shelter, being ready for bed (6)
I believe the answer is:
sleepy
'ready for bed' is the definition.
(I've seen this in another clue)
'agent accepts shelter' is the wordplay.
'agent' becomes 'spy' (spy is a kind of agent).
'accepts' is an insertion indicator.
'shelter' becomes 'lee' (as in lee side, the side sheltered from wind).
'spy' going around 'lee' is 'SLEEPY'.
'being' acts as a link.
(Other definitions for sleepy that I've seen before include "Feeling tired" , "Ready to turn in" , "Drowsy - without excitement" , "One of the Seven Dwarfs" , "Somnolent" .)