Small detached territory one’s rented out (5)
I believe the answer is:
islet
'small detached territory' is the definition.
(a small island, so detached from land)
'one's rented out' is the wordplay.
'one's' becomes 'I's' (Roman numeral).
'rented out' becomes 'let' (I've seen this in another clue).
'is'+'let'='ISLET'
(Other definitions for islet that I've seen before include "Small piece of land surrounded by water" , "Tiles to make a small island" , "A small piece of territory" , "One of Langerhans's" , "Very small area surrounded by water" .)