Something bad about good indicator (4)
I believe the answer is:
sign
'indicator' is the definition.
(I've seen this before)
'something bad about good' is the wordplay.
'something bad' becomes 'sin' (something morally bad).
'about' means one lot of letters goes inside another.
'good' becomes 'g' (abbreviation).
'sin' going around 'g' is 'SIGN'.
(Other definitions for sign that I've seen before include "Augury" , "Put one's name to a document" , "Autograph" , "Mark" , "Indication; symbol" .)