Stranger's daughter in river (5)
I believe the answer is:
odder
'stranger's' is the definition.
(I've seen this before)
'daughter in river' is the wordplay.
'daughter' becomes 'd'.
'in' means one lot of letters goes inside another.
'river' becomes 'oder' (River Oder in central Europe).
'd' put inside 'oder' is 'ODDER'.
(Other definitions for odder that I've seen before include "More unusual" , "more rum" , "Stranger" , "More strange or less even" , "More peculiar" .)