Possibly a fiver lost to a sweetheart long ago? (5,4)

I believe the answer is:

first love

I'm a little stuck... Click here to teach me more about this clue!

'a sweetheart long ago?' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.

'possibly a fiver lost' is the wordplay.
'possibly' indicates an anagram.
'fiver'+'lost'='fiverlost'
'fiverlost' is an anagram of 'FIRST LOVE'.

'to' acts as a link.

Can you help me to learn more?

Want a hint initially instead of a full solution? Install my app