Trade item again, instrument not about to be put in tin (4,2)

I believe the answer is:

sell on

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

'trade item again instrument not about to' is the definition.
I don't know anything about this answer so I can't tell whether this works.

'put in tin' is the wordplay.
'put' becomes 'ello' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'in' means one lot of letters goes inside another.
'tin' becomes 'Sn' (Sn is the chemical symbol for tin).
'ello' inserted within 'sn' is 'SELL ON'.

'be' acts as a link.

Can you help me to learn more?

I've seen this clue in The Guardian.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.