One got on ... or came down? (8)

I believe the answer is:

amounted

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'came down?' is the definition.
'amounted' can be an answer for 'came' (as in coming to or amounting to a certain amount). I'm not certain of the 'down?' bit.

'one got on' is the wordplay.
'one' becomes 'a' (a thing is one thing).
'got on' becomes 'mounted' ('mount' can be a synonym of 'get on').
'a'+'mounted'='AMOUNTED'

'or' is the link.

Can you help me to learn more?

I've seen this clue in The Independent.

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