Another exam, one taken in break (5)
I believe the answer is:
resit
'another exam' is the definition.
(sitting an exam again)
'one taken in break' is the wordplay.
'one' becomes 'i' (Roman numeral).
'taken in' is an insertion indicator.
'break' becomes 'rest' (a rest is taking a break).
'i' put into 'rest' is 'RESIT'.
(Other definitions for resit that I've seen before include "Retake exam" , "tested again" , "Take examination again" , "take another exam" , "second try at test" .)