Small body of land one's leased (5)
I believe the answer is:
islet
'small body of land' is the definition.
'one's leased' is the wordplay.
'one's' becomes 'is' (roman numeral).
'leased' becomes 'let'.
'is'+'let'='ISLET'
(Other definitions for islet that I've seen before include "Piece of land in e.g. river" , "Ait or eyot" , "detached territories?" , "Eyot, holm, inch" , "Little bit of land" .)