Part of mirror reflection shows up fault (5)
I believe the answer is:
error
'fault' is the definition.
(I know that fault can be written as error)
'part of mirror reflection shows up' is the wordplay.
'part of' says the answer is hidden in the clue.
'shows up' says the letters should be written backwards.
'rorre' can be found hidden inside 'mirror reflection'.
'rorre' backwards is 'ERROR'.
(Other definitions for error that I've seen before include "Mistake, lapse" , "Trial and ___" , "Howler" , "Solecism" , "Mistake or mishap" .)