One’s belt is loose, it’s alleged (10)
I believe the answer is:
ostensible
'alleged' is the definition.
(thesaurus)
'one's belt is loose' is the wordplay.
'loose' indicates anagramming the letters (letters are loosened from their original positions).
'ones'+'belt'+'is'='onesbeltis'
'onesbeltis' with letters rearranged gives 'OSTENSIBLE'.
'it's' acts as a link.
(Other definitions for ostensible that I've seen before include "Supposed" , "Manifest" , "Seeming" , "Apparently true" , "Apparent" .)