Current energy requirement essentially must stop (6)
I believe the answer is:
stream
'current' is the definition.
(I know that current can be written as stream)
'energy requirement essentially must stop' is the wordplay.
'energy' becomes 'steam' (I've seen this in another clue).
'requirement essentially' becomes 'r' (I can't justify this - if you can you should give a lot more credence to this answer).
'must stop' means one lot of letters goes inside another (stopping can mean blocking a gap).
'steam' placed around 'r' is 'STREAM'.
Can you help me to learn more?
(Other definitions for stream that I've seen before include "Running liquid" , "Current - file" , "Fly" , "group of pupils split by ability" , "Brook or small river" .)