Copper arresting unknown country fireman? (10)
I believe the answer is:
pyromaniac
'fireman?' is the definition.
(someone obsessed with fire)
'copper arresting unknown country' is the wordplay.
'copper' becomes 'PC' (police constable).
'arresting' indicates putting letters inside.
'unknown' becomes 'y' (algebra).
'country' becomes 'romania' (Romania is an example).
'y'+'romania'='yromania'
'pc' placed around 'yromania' is 'PYROMANIAC'.
(Other definitions for pyromaniac that I've seen before include "Arsonist" , "Fire-raiser" .)