Boob reflected in mirror, regrettably (5)
I believe the answer is:
error
'boob' is the definition.
(I've seen this before)
'reflected in mirror regrettably' is the wordplay.
'reflected' is a reversal indicator (letters reflected back).
'in' indicates a hidden word.
'rorre' can be found hidden inside 'mirror regrettably'.
'rorre' written backwards gives 'ERROR'.
(Other definitions for error that I've seen before include "Trial and ___" , "Boo-boo" , "Mistake, lapse" , "Gaffe" , "1 down" .)