Something only two can play (3)

I believe the answer is:

duo

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'play' is the definition.
The definition and answer can be both related to communication as well as being singular nouns.
Perhaps they are linked in a way I don't understand?

I don't understand the remainder of the clue.

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(Other definitions for duo that I've seen before include "Pair of musicians" , "Pair of singers" , "Item" , "Double act" , "Performing couple" .)

I've seen this clue in the Evening Standard.

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