Double spread turned into substantial publication (7)

I believe the answer is:

twosome

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'double' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for twosome that I've seen before include "Couple" , "Pair, duo" , "Item" , "match" , "game of golf" .)

I've seen this clue in The Telegraph.

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