A burning passion no longer? (3,5)
I believe the answer is:
'a burning passion no longer?' is the definition.
I can't judge whether this defines the answer.
'a burning passion' is the wordplay.
'a burning' becomes 'old' (I can't explain this - if you can you should believe this answer much more).
'passion' becomes 'flame' (as in the flame of a relationship).
Can you help me to learn more?
(Other definitions for old flame that I've seen before include "Ex-lover - the fire still burns" , "Former partner" , "Ex" , "Former sweetheart" , "Someone once loved" .)