Not a lot of land but somebody's leased it (5)
I believe the answer is:
islet
'somebody's leased it' is the definition.
I can't judge whether this definition defines the answer.
'not a lot of land' is the wordplay.
'not' becomes 'isl' (I can't justify this - if you can you should give a lot more credence to this answer).
'a lot of' means to remove the first letter.
'land' becomes 'get' (both can mean to secure or obtain).
'get' with its initial letter removed is 'et'.
'isl'+'et'='ISLET'
'but' is the link.
Can you help me to learn more?
(Other definitions for islet that I've seen before include "One of Langerhans's" , "Cay, for example" , "Eyot, holm, inch" , "Small landmass" , "Tiles to make a small island" .)