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' is an insertion indicator.
'his' becomes 'coper' (I can't explain this - if you can you should give a lot more credence to this answer).
'element?' becomes 'P' (P is an example).
'coper' going around '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?" .)