It's clever to have boots on sometimes (3)
I believe the answer is:
sly
'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" .)