A trying setback with the right conclusion (4)
I believe the answer is:
test
'a trying' is the definition.
(thesaurus)
'setback with the right conclusion' is the wordplay.
'setback' becomes 'tes' (I've seen this in other clues).
'with' means one lot of letters go next to another.
'conclusion' says to take the final letters.
The last letter of 'right' is 't'.
'tes'+'t'='TEST'
(Other definitions for test that I've seen before include "big game" , "Sample" , "Cricket match; try" , "Trial; Hampshire river" , "criterion" .)