Somebody's large whiskey -- one gallon! (6)
I believe the answer is:
bigwig
'somebody's' is the definition.
(in sense of somebody important)
'large whiskey one gallon' is the wordplay.
'large' becomes 'big' (synonyms).
'whiskey' becomes 'W' (phonetic alphabet: alpha, bravo, charlie etc.).
'one' becomes 'i' (Roman numeral).
'gallon' becomes 'g' (abbreviation for the unit of liquid).
'big'+'w'+'i'+'g'='BIGWIG'
(Other definitions for bigwig that I've seen before include "'Chief, top banana (6)'" , "important figure" , "Mogul" , "VIP with large toupee" , "Person of some importance" .)