Boob reflected in mirror, revealingly (5)
I believe the answer is:
error
'boob' is the definition.
(I know that boob can be written as error)
'reflected in mirror revealingly' is the wordplay.
'reflected' says the letters should be written in reverse (letters reflected back).
'in' indicates the answer is hidden within the clue.
'rorre' is hidden within 'mirror revealingly'.
'rorre' in reverse letter order is 'ERROR'.
(Other definitions for error that I've seen before include "Solecism" , "Fallacy, mistake" , "Blunder, slip" , "Slip, mistake" , "Miscarriage of justice" .)