… that's bad arrested by police (6)
I believe the answer is:
millet
'that's bad arrested by police' is the definition.
I can't judge whether this defines the answer.
'bad arrested by police' is the wordplay.
'bad' becomes 'ill' (associated in meaning).
'arrested by' is an insertion indicator (I've seen this in other clues).
'police' becomes 'met' (I've seen this in other clues).
'ill' put into 'met' is 'MILLET'.
Can you help me to learn more?
(Other definitions for millet that I've seen before include "Food-grain" , "Cereal grass for the French painter" , "Seed from cereal grasses - French painter, d.1875" , "Cereal plant producing large crop of small seeds" , "Cereal grass cultivated for animal fodder" .)