One’s leaving perfect bargain (4)
I believe the answer is:
deal
'bargain' is the definition.
(I know that bargain can be written as deal)
'one's leaving perfect' is the wordplay.
'one' becomes 'i' (Roman numeral).
'leaving' is a deletion indicator.
'perfect' becomes 'ideal' (similar in meaning).
'ideal' with 'i' removed is 'DEAL'.
(Other definitions for deal that I've seen before include "Bargain, contract" , "Buy and sell" , "Business arrangement in timber" , "Give several people a hand" , "A port; a (good) amount" .)