Convinced one it had been clinched (4,4)

I believe the answer is:

made sure

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

'convinced one it' is the definition.
I can't tell whether this definition defines the answer.

'been clinched' is the wordplay.
'been' becomes 'made' (making is a kind of being).
'clinched' becomes 'sure' (I can't explain this - if you can you should give a lot more credence to this answer).
'made'+'sure'='MADE SURE'

'had' is the link.

Can you help me to learn more?

I've seen this clue in the Evening Standard.

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.