Crossword Genius

Equivalent to light brown horse I'm obliged to break in (10)

Ross

I believe the answer is:

tantamount

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'equivalent' is the definition.
(I know that equivalent can be written as tantamount)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for tantamount that I've seen before include "Virtually equivalent (to)" , "Equivalent in seriousness (to)" , "Equivalent in value (to)" , "Equal in value" .)

I've seen this clue in The Independent.
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