Person one is no longer burning for (3,5)
I believe the answer is:
old flame
'no longer burning for' is the definition.
The definition suggests a singular noun which matches the answer.
'person one is' is the wordplay.
'person' becomes 'old' (old person is a kind of person).
'one is' becomes 'flame' (I can't justify this - if you can you should give a lot more credence to this answer).
'old'+'flame'='OLD FLAME'
Can you help me to learn more?
(Other definitions for old flame that I've seen before include "Someone once loved" , "Former sweetheart" , "Ex-lover - the fire still burns" , "Ex" , "Former lover" .)