Stopped copper meeting informer in Belfast (3,3)
I believe the answer is:
cut out
'stopped' is the definition.
'copper meeting informer in belfast' is the wordplay.
'copper' becomes 'cu' (chemical symbol).
'meeting' means one lot of letters go next to another.
'informer in belfast' becomes 'tout' (term used in Ireland for an informant).
'cu'+'tout'='CUT-OUT'
(Other definitions for cut out that I've seen before include "Excise" , "Cease operating" , "Cease to function, like car engine" , "Automatic safety circuit-breaker" , "25 [FAIL]" .)