Beyond belief originally, irregular accent (6)
I believe the answer is:
brogue
'accent' is the definition.
(brogue can mean a Scottish or Irish accent)
'beyond belief originally irregular' is the wordplay.
'beyond' says to put letters next to each other.
'originally' suggests taking the first letters.
'irregular' becomes 'rogue' (someone who doesn't conform to regular norms).
The initial letter of 'belief' is 'b'.
'b'+'rogue' is 'BROGUE'.
(Other definitions for brogue that I've seen before include "may be on a member's end" , "afoot?" , "Shoe - tongue" , "What might identify an Irishman" , "Footprint could be left by this" .)