It makes star pupils slip up? Rats! (8)
I believe the answer is:
reversal
'slip up? rats' is the definition.
The answer and definition can be both acts as well as being singular nouns.
Maybe there's an association between them I don't understand?
'it makes star pupils' is the wordplay.
I cannot really understand how this works, but
'it' could be 'sa' (old-fashioned abbreviation for sex appeal) and 'sa' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for reversal that I've seen before include "change of heart" , "A turning back" , "Turnabout" , "that makes no difference to mum or dad" , "that doesn't affect mum or dad" .)