Just heading off for exam (4)
I believe the answer is:
oral
'exam' is the definition.
(oral exam is a kind of exam)
'just heading off' is the wordplay.
'just' becomes 'moral' ('moral' can be a synonym of 'just').
'heading off' means to remove the first letter.
'moral' with its initial letter removed is 'ORAL'.
'for' is the link.
(Other definitions for oral that I've seen before include "Verbal" , "Of the voice" , "Spoken rather than written" , "Type of exam" , "Viva voce" .)