The vessels were travelling south (5)
I believe the answer is:
ewers
'the vessels' is the definition.
(ewer is a kind of vessel)
'were travelling south' is the wordplay.
'travelling' is an anagram indicator.
'south' becomes 's' ('S' can be a synonym of 'south').
'were' anagrammed gives 'ewer'.
'ewer'+'s'='EWERS'
(Other definitions for ewers that I've seen before include "Water jugs" , "Large jugs" , "containers" , "Pitchers" , "Jugs thrown into sewer" .)