One's levy halved for land offshore (4)
I believe the answer is:
isle
'land offshore' is the definition.
'one's levy halved' is the wordplay.
'one's' becomes 'is' (Roman numeral).
'halved' suggests halving the letters.
'levy' with half the letters taken is 'le'.
'is'+'le'='ISLE'
'for' is the link.
(Other definitions for isle that I've seen before include "Sheppey, perhaps" , "Detached territory" , "Little land" , "Rum, for example" , "you can't walk away from" .)