Many travelled south (6)
I believe the answer is:
droves
'many' is the definition.
(droves can mean large numbers of something)
'travelled south' is the wordplay.
'travelled' becomes 'drove' (travelled by car).
'south' becomes 'S' (synonyms).
'drove'+'s'='DROVES'
(Other definitions for droves that I've seen before include "Moving crowd of people" , "Very many, great numbers" , "Moving herds or flocks" .)