Setback by time trial (4)
I believe the answer is:
test
'trial' is the definition.
(I know that trial can be written as test)
'setback by time' is the wordplay.
'setback' becomes 'tes' (I've seen this in other clues).
'by' is a charade indicator (letters next to each other).
'time' becomes 't' (abbreviation).
'tes'+'t'='TEST'
(Other definitions for test that I've seen before include "catch trout in it?" , "Trial; Hampshire river" , "Trial; cricket match" , "Sample" , "criterion" .)