Crossword Genius

First to activate the pump (5)

Ross

I believe the answer is:

prime

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'first' is the definition.
(similar in meaning)

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for prime that I've seen before include "put in the picture" , "Best - get ready" , "Maybe eleven" , "Best quality, perhaps of beef" , "Chief or most important, like a minister" .)

I've seen this clue in The Irish Independent.
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