Sibling revised for second attempt! (6)
I believe the answer is:
resits
'sibling revised' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.
'second attempt' is the wordplay.
'second' becomes 're' (prefix meaning second in eg review, re-education).
'attempt' becomes 'sits' (I can't justify this - if you can you should believe this answer much more).
're'+'sits'='RESITS'
'for' acts as a link.
Can you help me to learn more?
(Other definitions for resits that I've seen before include "Exams taken for a second time" , "Does paper over" , "takes 15 [EXAM] again" , "Takes the examination again" .)