Partial arrest, ace copper says (5)
I believe the answer is:
reads
'says' is the definition.
('read' can be a synonym of 'say')
'partial arrest ace copper' is the wordplay.
'partial' becomes 'res' (I can't explain this - if you can you should give a lot more credence to this answer).
'arrest' is an insertion indicator.
'ace' becomes 'a' (abbreviation).
'copper' becomes 'd'.
'a'+'d'='ad'
'res' going around 'ad' is 'READS'.
Can you help me to learn more?
(Other definitions for reads that I've seen before include "Peruses" , "Studies (at university)" , "Scans" , "Looks at and undertsands the written word" , "Understands written language" .)