Cold drink, one originally left behind (5)
I believe the answer is:
lassi
'cold drink' is the definition.
(type of Indian drink)
'one originally left behind' is the wordplay.
'one' becomes 'i' (Roman numeral).
'originally' is a charade indicator (letters next to each other) (some letters go before others).
'left' becomes 'L' (common abbreviation).
'behind' becomes 'ass' (both can mean one's back side).
'l'+'ass'='lass'
'i' after 'lass' is 'LASSI'.
(Other definitions for lassi that I've seen before include "Indian dairy drink" , "creamy drink" , "drink in India" , "Indian yogurt drink" , "that Indian necks?" .)