Sold illicitly from sack by stage (7)
I believe the answer is:
bootleg
'sold illicitly' is the definition.
(I've seen this before)
'sack by stage' is the wordplay.
'sack' becomes 'boot' (I've seen this before).
'by' means one lot of letters go next to another.
'stage' becomes 'leg' ('leg' can be a synonym of 'stage').
'boot'+'leg'='BOOTLEG'
'from' is the link.
(Other definitions for bootleg that I've seen before include "Illegal (liquor/recording)" , "Pirated (copy)" , "Illegal music recording" , "Deal in illicit drink" , "obtained illegally" .)