Lump of something turning young horse's stomach (4)

I believe the answer is:

clot

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'lump' is the definition.
(I've seen this in another clue)

I can't explain the remainder of the clue.

(Other definitions for clot that I've seen before include "Semi-solid mass -- dunce" , "Thick lump - thick person" , "Foolish person" , "A lump in a blood vessel" , "(Of blood) coagulate" .)

I've seen this clue in The Independent.

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