As racist gets replaced, leader of cops is derisive (9)
I believe the answer is:
sarcastic
'derisive' is the definition.
(I've seen this before)
'as racist gets replaced leader of cops' is the wordplay.
'gets replaced' is an anagram indicator (letters placed in new positions).
'leader of' says to take the initial letters.
The initial letter of 'cops' is 'c'.
'as'+'racist'='asracist'
'asracist' is an anagram of 'sarcasti'.
'sarcasti'+'c'='SARCASTIC'
'is' acts as a link.
(Other definitions for sarcastic that I've seen before include "Sneering" , "Cruelly ironical" , "Using biting irony" , "Using mocking irony" , "Mockingly humorous" .)