Wrong girth, it's true (5)
I believe the answer is:
right
'true' is the definition.
(associated in meaning)
'wrong girth' is the wordplay.
'wrong' is an anagram indicator.
'girth' anagrammed gives 'RIGHT'.
'it's' is the link.
(Other definitions for right that I've seen before include "Most favourable" , "Morally correct" , "True - just claim" , "Quite proper" , "It's what you deserve" .)