Garment reflecting fashion seen across Northern Ireland (5)
I believe the answer is:
tunic
'garment' is the definition.
(historical type of garment)
'reflecting fashion seen across northern ireland' is the wordplay.
'reflecting' shows that the letters should be reversed in order.
'fashion' becomes 'cut' (cut is a kind of fashion).
'seen across' means one lot of letters goes inside another.
'northern ireland' becomes 'ni'.
'cut' back-to-front is 'tuc'.
'tuc' enclosing 'ni' is 'TUNIC'.
(Other definitions for tunic that I've seen before include "Clobber" , "Close-fitting jacket that's part of a uniform" , "Military uniform" , "(Policeman's) jacket" , "Shift" .)