Tired agent welcomed protection (6)
I believe the answer is:
sleepy
'tired' is the definition.
(thesaurus)
'agent welcomed protection' is the wordplay.
'agent' becomes 'spy' (spy is a kind of agent).
'welcomed' indicates putting letters inside (in sense of 'invited in').
'protection' becomes 'lee' (I've seen this before).
'spy' enclosing 'lee' is 'SLEEPY'.
(Other definitions for sleepy that I've seen before include "Soporific" , "Feeling like having a snooze" , "(Of place) without much activity" , "Quiet" , "Somnolent" .)