Detectives arrest emperor, turning radical (7)
I believe the answer is:
drastic
'radical' is the definition.
(I've seen this before)
'detectives arrest emperor turning' is the wordplay.
'detectives' becomes 'cid' (abbreviation for Criminal Investigation Department).
'arrest' means one lot of letters goes inside another.
'emperor' becomes 'tsar' (I've seen this before).
'turning' shows that the letters should be reversed in order.
'cid' placed around 'tsar' is 'citsard'.
'citsard' written backwards gives 'DRASTIC'.
(Other definitions for drastic that I've seen before include "Extreme - violent" , "Very severe or serious" , "Radical, far-reaching" , "Draconian" , "Extremely severe" .)