Were mistaken about the way to the river (5)
I believe the answer is:
weser
'the river' is the definition.
(German river)
'were mistaken about the way' is the wordplay.
'mistaken' indicates an anagram.
'about' means one lot of letters goes inside another.
'the way' becomes 's' (Compass direction - South).
'were' with letters rearranged gives 'weer'.
'weer' placed around 's' is 'WESER'.
'to' acts as a link.
(Other definitions for weser that I've seen before include "German runner" , "Er, sew up the German river" .)