During holiday, one further exam (5)
I believe the answer is:
resit
'further exam' is the definition.
(I've seen this before)
'during holiday one' is the wordplay.
'during' indicates putting letters inside.
'holiday' becomes 'rest' (rest can mean a holiday or break).
'one' becomes 'i' (Roman numeral).
'rest' enclosing 'i' is 'RESIT'.
(Other definitions for resit that I've seen before include "second try at test" , "Take examination again" , "later exam" , "second exam" , "Retake exam" .)