Slip up in bar or restaurant (5)
I believe the answer is:
error
'slip' is the definition.
(thesaurus)
'up in bar or restaurant' is the wordplay.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'in' indicates the answer is hidden within the clue.
'rorre' can be found hidden inside 'bar or restaurant'.
'rorre' written backwards gives 'ERROR'.
(Other definitions for error that I've seen before include "Trial and ___" , "Mistake or fault" , "Solecism" , "Slip-up" , "Blunder or mistake" .)