Beginners in key roles (7)

I believe the answer is:

openers

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'beginners in key roles' is the definition.
Although both the answer and definition are plural nouns, I cannot see how they can define each other.

This is the entire clue.

Can you help me to learn more?

(Other definitions for openers that I've seen before include "Keys" , "Batsmen going in first" , "Batsmen Nos 1 and 2" , "in demand at picnics" , "Two bats" .)

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

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