By law, liquor regularly taken with drug (4)
I believe the answer is:
iure
'by law' is the definition.
I don't know anything about this answer so I can't judge whether this works.
'liquor regularly taken with drug' is the wordplay.
'regularly taken' means one should take alternating letters (letters removed at regular intervals).
'with' means one lot of letters go next to another.
'drug' becomes 'e' (E is another name for ecstasy).
The alternate letters of 'liquor' are 'iur'.
'iur'+'e'='IURE'
Can you help me to learn more?
(Other definitions for iure that I've seen before include "by law by Cicero" , "by right" .)