Cross-country event I held in British and Irish territory getting curtailed (8)
I believe the answer is:
biathlon
'cross-country event' is the definition.
'i held in british and irish territory getting curtailed' is the wordplay.
'held in' is an insertion indicator.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'and' is a charade indicator (letters next to each other).
'irish territory' becomes 'athlone' (Irish town).
'getting curtailed' means to remove the last letter.
'athlone' with its final letter taken off is 'athlon'.
'b'+'athlon'='bathlon'
'i' put into 'bathlon' is 'BIATHLON'.
(Other definitions for biathlon that I've seen before include "Contest in two events, like cycling and running" , "winter sports event" , "Contest in two sports, like cycling and running" , "Contest in two sporting disciplines, say cycling and running" , "Shooting contest by skiers with rifles" .)