Boring king restrained outside (5)
I believe the answer is:
tired
'boring' is the definition.
(thesaurus)
'king restrained outside' is the wordplay.
'king' becomes 'R' (abbreviation of Latin rex).
'restrained' becomes 'tied' (tying is a kind of restraining).
'outside' indicates putting letters inside (some letters go outside others).
'r' placed within 'tied' is 'TIRED'.
(Other definitions for tired that I've seen before include "getting bored" , "Exhausted" , "In need of rest" , "Beat" , "Weary, fatigued" .)