A measure of good and bad (4)
I believe the answer is:
gill
'a measure' is the definition.
(gill is a kind of measure)
'good and bad' is the wordplay.
'good' becomes 'g' (abbreviation).
'and' is a charade indicator (letters next to each other).
'bad' becomes 'ill' ('ill' can be similar in meaning to 'bad'**).
'g'+'ill'='GILL'
'of' acts as a link.
(Other definitions for gill that I've seen before include "Fish's breathing apparatus" , "Breathing organ in fish" , "One quarter of a pint" , "Organ for breathing under water" , "Six nips" .)