Boring Romeo restrained outside (5)
I believe the answer is:
tired
'boring' is the definition.
(thesaurus)
'romeo restrained outside' is the wordplay.
'romeo' becomes 'r' (phonetic alphabet: alpha, bravo, charlie etc.).
'restrained' becomes 'tied' (tying is a kind of restraining).
'outside' indicates putting letters inside.
'r' inserted inside 'tied' is 'TIRED'.
(Other definitions for tired that I've seen before include "getting bored" , "Exhausted" , "Physically or mental weary" , "Played out" , "Depleted of energy" .)