A river disturbed a large group of vessels (6)
I believe the answer is:
armada
'large group of vessels' is the definition.
(a naval fleet)
'a river disturbed a' is the wordplay.
'river' becomes 'r' (abbreviation for river).
'disturbed' becomes 'mad' (synonyms).
'a'+'r'+'mad'+'a'='ARMADA'
(Other definitions for armada that I've seen before include "Fleet of armed ships" , "Spanish war fleet" , "Large flotilla" , "Navy" , "Large Spanish fleet" .)