Cause for cramming (4)
I believe the answer is:
exam
'cause for cramming' 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 "Written test" , "In brief, a test as in school" , "Set of questions evaluating skill or knowledge, in short" , "paper" , "Test (abbrev.)" .)