Sold illicitly from sack by stage (7)
I believe the answer is:
Here is my best explanation:
'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').
'from' is the link.
(Other definitions for bootleg that I've seen before include "illegal", "Deal in illicit drink", "Illegal (liquor/recording)", "Illicit (liquor?)", "Smuggle liquor", "Made or sold illegally", "Type of jeans", "Illegal alcohol during prohibition", "Distributed illegally" and "Illegally made (recording)".)