Girl is both good and bad (4)
I believe the answer is:
gill
'girl' is the definition.
'both good and bad' is the wordplay.
'both good' becomes 'g' (abbreviation. I am not sure about the 'both' bit.**).
'and' says to put letters next to each other.
'bad' becomes 'ill' ('ill' can be similar in meaning to 'bad').
'g'+'ill'='GILL'
'is' acts as a link.
(Other definitions for gill that I've seen before include "Breathing organ in fish" , "Fish's breathing organ" , "Small liquid measure" , "Respiratory organ of fish" , "A little drink" .)