Accounts book's advantage in long run (6)
I believe the answer is:
ledger
'accounts book's' is the definition.
(I've seen this in another clue)
'advantage in long run' is the wordplay.
'advantage' becomes 'edge' (having the edge is having an advantage).
'in' means one lot of letters goes inside another.
'long' becomes 'l'.
'run' becomes 'r' (cricket abbreviation).
'l'+'r'='lr'
'edge' put into 'lr' is 'LEDGER'.
(Other definitions for ledger that I've seen before include "In which accounts are entered" , "Accounts journal" , "Business record book" , "Heath -, Australian actor" , "Elderly resident" .)