Sister messed up second exams (6)
I believe the answer is:
resits
'second exams' is the definition.
(I've seen this before)
'sister messed up' is the wordplay.
'messed up' is an anagram indicator.
'sister' is an anagram of 'RESITS'.
(Other definitions for resits that I've seen before include "Takes the examination again" , "Does paper over" , "takes 15 [EXAM] again" , "Exams taken for a second time" .)