Record-holder left border? Right! (6)
I believe the answer is:
ledger
'record-holder' is the definition.
(I've seen this before)
'left border? right' is the wordplay.
'left' becomes 'l' (common abbreviation).
'border?' becomes 'edge' (synonyms).
'right' becomes 'r' (common abbreviation).
'l'+'edge'+'r'='LEDGER'
(Other definitions for ledger that I've seen before include "In which accounts are entered" , "Elderly resident" , "Accounting journal" , "Heath -, Australian actor" , "Old accounts book" .)