Severe test — exam without notes (6)
I believe the answer is:
Here is my best explanation:
'severe test' is the definition.
(I've seen this before)
'exam without notes' is the subsidiary indication.
'exam' becomes 'oral' (spoken academic test).
'without' is an insertion indicator.
'notes' becomes 'de' (**).
'oral' placed around 'de' is 'ordeal'.
(Other definitions for ordeal that I've seen before include "Distressing experience", "Prolonged distressing experience", "Harrowing experience", "Trial; nightmare", "Difficult experience that tests endurance", "Severe trial", "Tribulation", "Difficult and testing experience", "Torment" and "A difficult and harrowing experience".)