A river -- round the bend a large group of vessels (6)
I believe the answer is:
armada
'large group of vessels' is the definition.
(a naval fleet)
'a river round the bend a' is the wordplay.
'river' becomes 'r' (abbreviation for river).
'round the bend' becomes 'mad' (round the bend can informally mean mad).
'a'+'r'+'mad'+'a'='ARMADA'
(Other definitions for armada that I've seen before include "Old Spanish fleet" , "A great fleet - Spain had one" , "Large Spanish fleet" , "A great fleet of warships" , "Large flotilla" .)