Policeman in his element? (6)
I believe the answer is:
copper
'policeman' is the definition.
(copper can informally mean a police officer)
'in his element?' is the wordplay.
'in' means one lot of letters goes inside another.
'his' becomes 'coper' (I can't justify this - if you can you should give a lot more credence to this answer).
'element?' becomes 'P' (P is an example).
'coper' enclosing 'p' is 'COPPER'.
Can you help me to learn more?
(Other definitions for copper that I've seen before include "Busy" , "Metallic element and British bobby" , "English policeman" , "Metal for slang policeman" , "Metal - washing-machine?" .)