Tired agent takes shelter inside (6)
I believe the answer is:
sleepy
'tired' is the definition.
(falling asleep)
'agent takes shelter inside' is the wordplay.
'agent' becomes 'spy' (spy is a kind of agent).
'takes' indicates putting letters inside.
'shelter inside' becomes 'lee' (as in lee side, the side sheltered from wind. I am not sure about the 'inside' bit.).
'spy' placed around 'lee' is 'SLEEPY'.
Can you help me to learn more?
(Other definitions for sleepy that I've seen before include "Dozy" , "Drowsy; indolent" , "Soporific" , "in retirement?" , "Quiet" .)