Great at organising an event (7)
I believe the answer is:
regatta
'an event' is the definition.
(regatta is a kind of event)
'great at organising' is the wordplay.
'organising' is an anagram indicator.
'great'+'at'='greatat'
'greatat' is an anagram of 'REGATTA'.
(Other definitions for regatta that I've seen before include "Boat-racing meeting" , "It could involve a lot of rowing" , "Yacht-racing event" , "set of marine races?" , "Display of boats" .)