Sale's not open? (5,3,7)
I believe the answer is:
under the counter
'open?' is the definition.
Both the definition and answer are adjectives. Maybe you can see a link between them that I can't see?
'sale's not' is the wordplay.
I cannot really see how this works, but
'not' could be 'nt' (synonyms) and 'nt' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for under the counter that I've seen before include "Serving only favoured customers" , "Secretly" , "Surreptitiously" , "avoiding controls" , "Sold illicitly" .)