Clothier in Rialto ruined (6)
I believe the answer is:
tailor
'clothier' is the definition.
(thesaurus)
'rialto ruined' is the wordplay.
'ruined' indicates an anagram.
'rialto' is an anagram of 'TAILOR'.
'in' acts as a link.
(Other definitions for tailor that I've seen before include "Fit" , "Maker of men's suits" , "One makes fitted clothes" , "Maker of garments" , "One who makes garments" .)