Left side, right book (6)
I believe the answer is:
ledger
'book' is the definition.
(I know that book can be written as ledger)
'left side right' is the wordplay.
'left' becomes 'l' (common abbreviation).
'side' becomes 'edge' (edge is a kind of side).
'right' becomes 'r' (common abbreviation).
'l'+'edge'+'r'='LEDGER'
(Other definitions for ledger that I've seen before include "Elderly resident" , "record-holder" , "In which accounts are entered" , "Business record book" , "Account book" .)