Crossword Genius

Highly improbable (4)

Ross

I believe the answer is:

tall

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'improbable' is the definition.
(synonyms)

I don't understand how the rest of the clue works.

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(Other definitions for tall that I've seen before include "Towering" , "Bigger than average" , "Remarkable" , "Of great height" , "Unbelievable" .)

I've seen this clue in the Evening Standard.
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