Working at great sporting event (7)
I believe the answer is:
regatta
'sporting event' is the definition.
(I've seen this before)
'working at great' is the wordplay.
'working' indicates anagramming the letters.
'at'+'great'='atgreat'
'atgreat' is an anagram of 'REGATTA'.
(Other definitions for regatta that I've seen before include "Number of yachts in sporting event" , "Yacht-race event" , "Boat meeting (at Henley, say)" , "Rowing, sailing, gala" , "Race meeting for boats" .)