Tests: exams including endless study (7)
I believe the answer is:
'tests' is the definition.
(I have seen 'difficult tests' mean 'ordeals' so perhaps 'tests' could also mean 'ordeals')
'exams including endless study' is the wordplay.
'exams' becomes 'orals' (oral is a kind of exam).
'including' means one lot of letters goes inside another.
'endless' means to remove the last letter.
'study' becomes 'den' (term for a study or office).
'den' with its last letter taken off is 'de'.
'orals' enclosing 'de' is 'ORDEALS'.
(Other definitions for ordeals that I've seen before include "Difficult experiences, hard times" , "Trying experiences" , "difficult tests" , "Testing experiences" , "Tribulations" .)