Not all candidates take an examination (4)
I believe the answer is:
test
'examination' is the definition.
(I've seen this before)
'not all candidates take an' is the wordplay.
'not all' indicates the answer is hidden within the clue.
'TEST' is hidden within 'candidates take an'.
(Other definitions for test that I've seen before include "Try, examine critically" , "Exam to play international cricket" , "Cricket match; exam" , "Assay" , "Audition" .)