Small detached territory where one’s allowed to meet (5)
I believe the answer is:
islet
'small detached territory' is the definition.
(a small island, so detached from land)
'one's allowed to meet' is the wordplay.
'one's' becomes 'I's' (Roman numeral).
'allowed' becomes 'let' (to let is to allow or permit).
'to meet' says to put letters next to each other.
'is'+'let'='ISLET'
'where' acts as a link.
(Other definitions for islet that I've seen before include "Small piece of land surrounded by water" , "detached territories?" , "Small landmass" , "Detached territory" , "One of Langerhans's" .)