Constantly changing Rising lacks a leader (9)

I believe the answer is:

evolution

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'leader' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for evolution that I've seen before include "Darwinian process" , "Darwinian theory" , "Development - progress" , "Darwin's theory" , "Gradual change" .)

I've seen this clue in the Irish Times.

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