On a river, a weir capsized some ships (6)
I believe the answer is:
armada
'some ships' is the definition.
(group of ships)
'on a river a weir capsized' is the wordplay.
'on' says to put letters next to each other.
'river' becomes 'r' (abbreviation for river).
'weir' becomes 'dam' (I've seen this in another clue).
'capsized' says the letters should be written in reverse (in a down clue, the word turns upside down).
'a'+'dam'='adam'
'adam' reversed gives 'mada'.
'a' put next to 'r' is 'ar'.
'ar'+'mada'='ARMADA'
(Other definitions for armada that I've seen before include "lot of craft" , "group of ships" , "Fleet of armed ships" , "Large number of ships (Spanish?)" , "sea-borne unit" .)