Former sweetheart (3,5)
I believe the answer is:
'former sweetheart' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for old flame that I've seen before include "still 1 across [ALIGHT], it seems" , "Someone once loved" , "Ex-lover - the fire still burns" , "Ex" , "Former partner" .)