Key one's rented (5)
I believe the answer is:
islet
'key' is the definition.
(both can mean a small island)
'one's rented' is the wordplay.
'one's' becomes 'i's' (Roman numeral).
'rented' becomes 'let' (I've seen this before).
'is'+'let'='ISLET'
(Other definitions for islet that I've seen before include "detached territories?" , "Small landmass" , "Stile (anag)" , "named after Langerhans?" , "Piece of land in e.g. river" .)