A city rent is ridiculous? Sure thing (9)
I believe the answer is:
certainty
'sure thing' is the definition.
(I know that sure thing can be written as certainty)
'a city rent is ridiculous?' is the wordplay.
'is ridiculous?' is an anagram indicator.
'a'+'city'+'rent'='acityrent'
'acityrent' anagrammed gives 'CERTAINTY'.
(Other definitions for certainty that I've seen before include "Sureness" , "Banker" , "Cinch" , "Something definite" , "State of being sure" .)