"Why not __?" (4)
'why not' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for sure that I've seen before include "stable" , "Completely certain" , "Certain, confident" , "of course" , "Feeling no doubt" .)
(Other definitions for lets that I've seen before include "Leases" , "Yes" , "Allows or causes to" , "Rents out or allows" , "Permits" .)
(Other definitions for both that I've seen before include "couple" , "Each of two (alternatives)" , "Two considered together" , "The one and the other" , "The two (things)" .)
'why not?' is the definition.
(I've seen this in another clue)
This is all the clue.