Crossword Genius

Naturally it can at length be crushing (3)

Ross

I believe the answer is:

boa

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'crushing' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.

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

Can you help me to learn more?

(Other definitions for boa that I've seen before include "Fluffy scarf of feathers" , "One who puts the squeeze on" , "Large constrictor snake" , "Wrap" , "strangler" .)

I've seen this clue in the Evening Standard.
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