One appearing in Trilby wasnt! (7)

I believe the answer is:

hatless

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'one' is the definition.
Both the answer and definition are adjectives. 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 hatless that I've seen before include "Without a titfer" , "Uncovered" .)

I've seen this clue in The Irish Independent.

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