Approximate length of an informal adieu (6)

'an informal adieu' is the definition.
I know nothing about this answer so I cannot judge whether this works.

'approximate length of' is the wordplay.
I cannot really understand how this works, but
'length' could be 'l' (abbreviation in geometry) and 'l' is located in the answer.
'of' could be 'o' and 'o' is found within the answer.
The remaining letters 'song' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?