They show up fancy muscles (9)

I believe the answer is:

flashguns

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'they' is the definition.
Both the answer and definition are plural nouns.
Perhaps there's a link between them I don't understand?

'show up fancy muscles' is the wordplay.
'show' becomes 'flash' (flashing is a kind of showing).
'up fancy muscles' becomes 'guns' (I can't justify this - if you can you should give a lot more credence to this answer).
'flash'+'guns'='FLASHGUNS'

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I've seen this clue in The Guardian.

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