Desires to have apprentice replace new young horses (9)

I believe the answer is:

yearlings

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'young horses' is the definition.
(I've seen this before)

I can't explain the rest of the clue.

(Other definitions for yearlings that I've seen before include "young runners" , "Racehorses in their second year" , "Young horses" .)

I've seen this clue in The Guardian.

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