Gathering police must conserve energy (4)
I believe the answer is:
meet
'gathering' is the definition.
(thesaurus)
'police must conserve energy' is the wordplay.
I cannot really understand how this works, but
'energy' could be 'e' (physics symbol as in E=mc2) and 'e' is found within the answer.
'police' could be 'met' (London Metropolitan Police) and 'met' is found in the leftover letters.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for meet that I've seen before include "See 24" , "Appropriate" , "club" , "Come upon" , "Join" .)