A rent isn't modified in passing (9)
I believe the answer is:
transient
'passing' is the definition.
(I know that passing can be written as transient)
'a rent isn't modified' is the wordplay.
'modified' indicates anagramming the letters.
'a'+'rent'+'isnt'='arentisnt'
'arentisnt' with letters rearranged gives 'TRANSIENT'.
'in' acts as a link.
(Other definitions for transient that I've seen before include "Here today, gone tomorrow" , "Lasting a short time" , "Lasting little time" , "on the move" , "fugitive" .)