Heavyweight notorious rapper's whiskey (one gallon) (6)
I believe the answer is:
bigwig
'heavyweight' is the definition.
(I've seen this in another clue)
'notorious rapper's whiskey one gallon' is the wordplay.
'notorious rapper' becomes 'big' (I can't justify this - if you can you should believe this answer much more).
'whiskey' becomes 'W' (alpha bravo charlie delta ...).
'one' becomes 'i' (Roman numeral).
'gallon' becomes 'g' (abbreviation for the unit of liquid).
'big'+'w'+'i'+'g'='BIGWIG'
Can you help me to learn more?
(Other definitions for bigwig that I've seen before include "Mogul" , "Important person, with large, bald head?" , "VIP with large toupee" , "Dignitary (sl.)" , "What may lie ahead largely" .)