Crossword Genius

Under cover, with leg next to a particular duvet where length is key (2,3,5)

Ross

I believe the answer is:

on the quiet

'under cover' is the definition.

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

(Other definitions for on the quiet that I've seen before include "Unobtrusively" , "Secretly" , "Surreptitiously" .)

I've seen this clue in The Times.
Want a hint initially instead of a full solution? Install my app