One's belt is loose, that's apparent (10)
I believe the answer is:
ostensible
'apparent' is the definition.
(I know that apparent can be written as ostensible)
'one's belt is loose' is the wordplay.
'loose' indicates an anagram (letters are loosened from their original positions).
'ones'+'belt'+'is'='onesbeltis'
'onesbeltis' with letters rearranged gives 'OSTENSIBLE'.
'that's' acts as a link.
(Other definitions for ostensible that I've seen before include "Supposed" , "Manifest" , "Professed" , "Apparently true" , "Seeming" .)