Nine is the most popular time for broadcasters (5)

I believe the answer is:

prime

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'most popular time for broadcasters' is the definition.
(as in a prime time television programme)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for prime that I've seen before include "Prepare to fire - first" , "Original" , "Main - charge" , "Number such as two, three, five, seven, etc" , "Best - get ready" .)

I've seen this clue in The Guardian.

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