Small piece of land isn't available? (5)
I believe the answer is:
islet
'small piece of land' is the definition.
(I've seen this before)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for islet that I've seen before include "detached territories?" , "Eyot, holm, inch" , "Ait or key" , "Very small area surrounded by water" , "Piece of land in e.g. river" .)