Storyteller at corn exchange over regret (9)
I believe the answer is:
raconteur
'storyteller' is the definition.
(I know that raconteur is a type of storyteller)
'at corn exchange over regret' is the wordplay.
'exchange' indicates anagramming the letters.
'over' says the letters should be written in reverse.
'regret' becomes 'rue' (to rue something is to regret it).
'rue' in reverse letter order is 'eur'.
'at'+'corn'='atcorn'
'atcorn' with letters rearranged gives 'racont'.
'racont'+'eur'='RACONTEUR'
(Other definitions for raconteur that I've seen before include "One who tells amusing tales" , "Teller of anecdote" , "He's good at telling stories" , "A skilled story teller" , "One used to telling stories" .)