One who watches 'The Sixth Jug' (6)
I believe the answer is:
viewer
'one who watches' is the definition.
(I've seen this before)
'the sixth jug' is the wordplay.
'the sixth' becomes 'VI' (Roman numerals).
'jug' becomes 'ewer' (type of vessel).
'vi'+'ewer'='VIEWER'
(Other definitions for viewer that I've seen before include "He surveys, or watches TV" , "Onlooker" , "Spectator" , "One seeing" , "One watching, eg, TV" .)