Relax when one’s coming out of repeated exam (4)
I believe the answer is:
rest
'relax' is the definition.
(to relax is to have a rest)
'one's coming out of repeated exam' is the wordplay.
'one' becomes 'i' (Roman numeral).
'coming out of' indicates named letters should be taken away.
'repeated exam' becomes 'resit' (sitting an exam again).
'resit' with 'i' removed is 'REST'.
'when' acts as a link.
(Other definitions for rest that I've seen before include "Take it easy, have a break" , "'Relax, pause from work (4)'" , "Repose, others" , "Remain" , "Stay" .)