Master switch for the current (6)
I believe the answer is:
stream
'the current' is the definition.
('stream' can be a synonym of 'current')
'master switch' is the wordplay.
'switch' indicates an anagram.
'master' with letters rearranged gives 'STREAM'.
'for' acts as a link.
(Other definitions for stream that I've seen before include "Small narrow flow" , "group of schoolchildren" , "One small river" , "Natural body of running water" , "Constant flow of liquid" .)