Great at organising sporting event (7)
I believe the answer is:
regatta
'sporting event' is the definition.
(I've seen this before)
'great at organising' is the wordplay.
'organising' indicates an anagram.
'great'+'at'='greatat'
'greatat' anagrammed gives 'REGATTA'.
(Other definitions for regatta that I've seen before include "Boating event" , "Boat-race meeting" , "Yacht-race event" , "races at sea" , "Sporting event with boat or yacht races" .)