Crossword Genius

It's clever to have boots on sometimes (3)

Ross

I believe the answer is:

sly

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'it's clever' is the definition.
(sly means clever or cunning)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for sly that I've seen before include "Cunning; surreptitious" , "'Crafty, cunning (3)'" , "'Cunning, wily (3)'" , "Wily, shifty" , "Doing things in unpleasant and secret way" .)

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