Escapist novel union hid, mostly (7)

I believe the answer is:

houdini

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'escapist' is the definition.
Both the definition and answer are singular nouns.
Maybe there's a link between them I don't understand?

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for houdini that I've seen before include "American escapologist, d. 1926" , "Harry -, US escapologist" , "World-famous escapologist" , "Showman" , "Harry . . . . . . ., famed American escapologist" .)

I've seen this clue in The Times.

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