Enthusiastic patron of local obstacle race (6)
I believe the answer is:
'enthusiastic patron of local' is the definition.
(barfly can mean someone who drinks in a bar)
'obstacle race' is the wordplay.
'obstacle' becomes 'bar' (bar can mean an obstacle or blockage).
'race' becomes 'fly' (I've seen this in other clues).
(Other definitions for barfly that I've seen before include "pub regular" , "frequenter of pubs" .)