Like a first-class, though useless, gong (10)

I believe the answer is:

unbeatable

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'like' is the definition.
Both the answer and definition are adjectives. Perhaps you can see an association between them that I don't see?

I cannot understand how the rest of the clue works.

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(Other definitions for unbeatable that I've seen before include "best" , "Invincible" .)

I've seen this clue in the Evening Standard.

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