Crossword Genius

One thing you still gain when on a decline (7)

Ross

I believe the answer is:

impetus

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

'one thing' is the definition.
The definition and answer can be both acts as well as being singular nouns.
Perhaps there's an association between them I don't understand?

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for impetus that I've seen before include "Driving force, momentum" , "Imputes to force with which a body moves" , "Drive" , "Catalyst" , "influence" .)

I've seen this clue in The Globe and Mail.
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