Bears twin daughters, one dunked in river (7)
I believe the answer is:
teddies
'bears' is the definition.
(teddy bears)
'twin daughters one dunked in river' is the wordplay.
'twin daughters' can mean two replacements for 'daughters'.
'daughters' becomes 'd' (genealogical abbreviation for daughters).
'daughters' becomes 'd' (genealogical abbreviation for daughters).
'one' becomes 'i' (Roman numeral).
'dunked in' is an insertion indicator.
'river' becomes 'tees' (River Tees).
'd'+'d'+'i'='ddi'
'ddi' put inside 'tees' is 'TEDDIES'.
(Other definitions for teddies that I've seen before include "Soft toy bears" , "Children's toy bears" , "Undergarments and children's toys" .)