Tests: exams including endless study (7)
I believe the answer is:
ordeals
'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" .)