Irish region in Istanbul's territory (6)
I believe the answer is:
ulster
'irish region' is the definition.
'ulster' can be an answer for 'region' (I have seen 'Region of Ireland ' mean 'ulster' so perhaps 'region' could also mean 'ulster'). I also know that 'irish' relates to this answer.
'in istanbul's territory' is the wordplay.
'in' indicates the answer is hidden within the clue.
'ULSTER' is hidden within 'istanbuls territory'.
(Other definitions for ulster that I've seen before include "Loose overcoat - result (anag)" , "Loose, long heavy overcoat - rustle (anag)" , "old province" , "Cloak for an Irish province" , "Long loose overcoat, from the province?" .)