Biscuit a girl replaced with bad one (9)
I believe the answer is:
garibaldi
'biscuit' is the definition.
(I've seen this before)
'a girl replaced with bad one' is the wordplay.
'replaced with' indicates an anagram.
'one' becomes 'i' (roman numeral).
'a'+'girl'='agirl'
'agirl' mixed up with 'bad' makes 'garibald'.
'garibald'+'i'='GARIBALDI'
(Other definitions for garibaldi that I've seen before include "Revolutionary leader" , "Lady's blouse" , "girl's blouse!" , ""Squashed fly" biscuit?" , "Italian hero, who led his 'Thousand Redshirts' against the Austrians, d. 1882" .)