Setback confused several crossing river (8)
I believe the answer is:
reversal
'setback' is the definition.
(I know that setback can be written as reversal)
'confused several crossing river' is the wordplay.
'confused' is an anagram indicator.
'crossing' means one lot of letters goes inside another.
'river' becomes 'r' (abbreviation for river).
'several' anagrammed gives 'revesal'.
'revesal' placed around 'r' is 'REVERSAL'.
(Other definitions for reversal that I've seen before include "U-turn" , "A turning backwards" , "Change of fortune" , "that makes no difference to mum or dad" , "Volte-face" .)