In horror, reporter turned up mistake (5)
I believe the answer is:
error
'mistake' is the definition.
(I know that mistake can be written as error)
'in horror reporter turned up' is the wordplay.
'in' indicates the answer is hidden within the clue.
'turned up' shows that the letters should be reversed in order.
'rorre' is hidden within 'horror reporter'.
'rorre' back-to-front is 'ERROR'.
(Other definitions for error that I've seen before include "Mistake or fault" , "Howler" , "Mistake or mishap" , "Mistake from terrorist" , "Slip, mistake" .)