Reason to study
(Other definitions for test that I've seen before include "Try, examine critically" , "Dip one's toe in" , "Assess - examination" , "Ward off" , "Analyse" .)
'reason to study' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for exam that I've seen before include "Students sit it" , "School test (colloq.)" , "'Test, in short (4)'" , "End-of-course assessment" , "Something to sit" .)