Crossword Genius

From behind, all bottoms presumably perfect (4,2)

Ross

I believe the answer is:

spot on

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

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

Can you help me to learn more?

(Other definitions for spot on that I've seen before include "great accuracy" , "Excellent" , "Absolutely right" , "Precise" , "Quite right" .)

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