Setback confused several on Right (8)
I believe the answer is:
reversal
'setback' is the definition.
(I know that setback can be written as reversal)
'confused several on right' is the wordplay.
'confused' is an anagram indicator.
'on' means one lot of letters goes inside another (as in clothing 'on' a person).
'right' becomes 'r' (common abbreviation).
'several' with letters rearranged gives 'revesal'.
'revesal' enclosing 'r' is 'REVERSAL'.
(Other definitions for reversal that I've seen before include "Change of fortune" , "change for the worse" , "A turning back" , "Turnaround - defeat" , "complete change" .)