Big test; torture (6)
I believe the answer is:
ordeal
This clue is a double definition.
'big test' is the first definition.
(I've seen this in another clue)
'torture' is the second definition.
(I know that ordeal is a type of torture)
(Other definitions for ordeal that I've seen before include "A difficult and harrowing experience" , "Difficult experience that tests endurance" , "Difficult experience, a tough time" , "Disturbing experience" , "Severe or trying experience" .)