Sell illicitly from sack by stage (7)
I believe the answer is:
bootleg
'sell illicitly' is the definition.
'bootleg' can be an answer for 'sell' (bootlegging is a kind of selling). I am not certain of the 'illicitly' bit.
'sack by stage' is the wordplay.
'sack' becomes 'boot' (I've seen this before).
'by' says to put letters next to each other.
'stage' becomes 'leg' (a leg is a stage of a competition).
'boot'+'leg'='BOOTLEG'
'from' is the link.
Can you help me to learn more?
(Other definitions for bootleg that I've seen before include "Illegal product" , "Illegally made recordings" , "Deal in illicit drink" , "suitable for visit to shebeen?" , "Type of jeans" .)