Carouse as of old, hiding whiskey in mug, say (6)
I believe the answer is:
assail
'carouse as of old hiding whiskey' is the definition.
I can't judge whether this definition defines the answer.
'mug say' is the wordplay.
'mug' becomes 'ass' (I've seen this before).
'say' becomes 'ail' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'ass'+'ail'='ASSAIL'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for assail that I've seen before include "Beset" , "Attack, lash out" , "Make violent attack on" , "Trouble" , "Slate" .)