Rearranged Banbury's leading fund-raising event (5,3,3,4)
I believe the answer is:
bring and buy sale
'fund-raising event' is the definition.
(I've seen this before)
'rearranged banbury's leading' is the wordplay.
'rearranged' indicates anagramming the letters.
'banburys'+'leading'='banburysleading'
'banburysleading' is an anagram of 'BRING-AND-BUY SALE'.
(Other definitions for bring and buy sale that I've seen before include "charity event" , "fund-raising event" , "Fund-raiser" .)