Suspect copper's arrested by brilliant old copper (7)
I believe the answer is:
accused
'suspect' is the definition.
('accused' can be a synonym of 'suspect')
'copper's arrested by brilliant old copper' is the wordplay.
'copper's' becomes 'cus' (chemical symbol).
'arrested by' means one lot of letters goes inside another (I've seen this in other clues).
'brilliant' becomes 'ace' (I've seen this before).
'old copper' becomes 'd'.
'cus' placed inside 'ace' is 'accuse'.
'accuse'+'d'='ACCUSED'
(Other definitions for accused that I've seen before include "Defendant appearing on a criminal charge" , "Incriminated" , "One tried" , "Charged with an offence" , "Person being charged" .)