Mock law, having one upsetting detectives (8)
I believe the answer is:
ridicule
'mock' is the definition.
(ridiculing is a kind of mocking)
'law having one upsetting detectives' is the wordplay.
'law' becomes 'rule' (rule is a kind of law).
'having' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'upsetting' shows that the letters should be reversed in order.
'detectives' becomes 'CID' (abbreviation for Criminal Investigation Department).
'cid' written backwards gives 'dic'.
'i'+'dic'='idic'
'rule' placed around 'idic' is 'RIDICULE'.
(Other definitions for ridicule that I've seen before include "Meek" , "Mock, make fun of." , "satire" , "guy" , "Pour scorn on" .)