Is permitted into the small piece of land (5)
I believe the answer is:
islet
'the small piece of land' is the definition.
(small island)
'is permitted' is the wordplay.
'permitted' becomes 'let' (to let something happen is to permit it).
'is'+'let'='ISLET'
'into' acts as a link.
(Other definitions for islet that I've seen before include "named after Langerhans?" , "Detached bit of land" , "Ait or key" , "Tiles (anag.)" , "Ait or eyot" .)