Member of aristocracy leads part of NI in preparation for event (9)
I believe the answer is:
countdown
'preparation for event' is the definition.
(the countdown to an event)
'member of aristocracy leads part of ni' is the wordplay.
'member of aristocracy' becomes 'count' (count is a type of aristocrat).
'leads' means one lot of letters go next to another.
'part of ni' becomes 'down' (County Down, part of Northern Ireland).
'count'+'down'='COUNTDOWN'
'in' is the link.
(Other definitions for countdown that I've seen before include "Three, two, one - launch spacecraft" , "5-4-3-2-1" , "The reckoning" , "Final timing for launching spacecraft" , "prelude to firing" .)