Very rare comic (9)

I believe the answer is:

priceless

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'very rare comic' is the definition.
Both the answer and definition are adjectives. Perhaps there's an association between them I don't understand?

This is all the clue.

Can you help me to learn more?

(Other definitions for priceless that I've seen before include "Very precious; absurd" , "Invaluable - very funny" , "Wonderful" , "of inestimable worth" , "Rich" .)

I've seen this clue in The Telegraph.

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