Spectators seeing six departing vessels (5)
I believe the answer is:
ewers
'vessels' is the definition.
(ewer is a kind of vessel)
'spectators seeing six departing' is the wordplay.
'spectators seeing' becomes 'viewers' (people seeing things).
'six' becomes 'VI' (Roman numerals).
'departing' is a deletion indicator.
'viewers' with 'vi' taken away is 'EWERS'.
(Other definitions for ewers that I've seen before include "Pitchers" , "Water jugs" , "Vase-shaped jugs" , "Vessels with handle and spout" , "Large jugs" .)