Describing uncovered wheels, we hear, that won't get worn out (8)

I believe the answer is:

tireless

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'out' is the definition.
Both the answer and definition are adjectives. Maybe they are linked in a way I don't understand?

I don't understand the rest of the clue.

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(Other definitions for tireless that I've seen before include "Full of stamina" , "Never grows weary, inexhaustible" , "Having great energy" , "Indefatigable, unflagging" , "Full of energy" .)

I've seen this clue in The Irish Independent.

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