One thing you still gain when on a decline (7)
I believe the answer is:
impetus
'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" .)