Potion does not contain top shelf liquor (7)
I believe the answer is:
nostrum
'potion' is the definition.
(I have seen 'secret potion' mean 'nostrum' so perhaps 'potion' could also mean 'nostrum')
'not contain top shelf liquor' is the wordplay.
'contain' is an insertion indicator.
'top' suggests taking the first letters (in a down clue, the first letter is top).
'liquor' becomes 'rum' (rum is a kind of liquor).
The first letter of 'shelf' is 's'.
'not' going around 's' is 'nost'.
'nost'+'rum'='NOSTRUM'
'does' is the link.
(Other definitions for nostrum that I've seen before include "Quack medicine - run most (anag)" , "secret potion" , "Tonic" , "Must Ron take a quack remedy?" , "Some think it helps" .)