Crossword Genius

Food that a racehorse needs (4)

Ross

I believe the answer is:

tack

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

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for tack that I've seen before include "Small broad-headed nail" , "Alter course - riding gear" , "Rubbishy things" , "main course?" , "Small nail with broad head" .)

I've seen this clue in The Telegraph.
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