Mistake admitted by juror, really upset (5)
I believe the answer is:
error
'mistake' is the definition.
('error' can be a synonym of 'mistake')
'admitted by juror really upset' is the wordplay.
'admitted by' indicates the answer is hidden within the clue.
'upset' shows that the letters should be reversed in order.
'rorre' is hidden within 'juror really'.
'rorre' in reverse letter order is 'ERROR'.
(Other definitions for error that I've seen before include "Way of operating" , "Faulty judgment" , "1 down" , "heresy" , "Solecism" .)