The correct girth, maybe (5)
I believe the answer is:
right
'the correct' is the definition.
(thesaurus)
'girth maybe' is the wordplay.
'maybe' indicates an anagram.
'girth' is an anagram of 'RIGHT'.
(Other definitions for right that I've seen before include "Morally correct" , "It's what you deserve" , "licence" , "Due" , "Accurate" .)