Master-switch for the current (6)
I believe the answer is:
stream
'the current' is the definition.
(synonyms)
'master-switch' is the wordplay.
'switch' indicates an anagram.
'master' anagrammed gives 'STREAM'.
'for' is the link.
(Other definitions for stream that I've seen before include "schoolchildren grouped together" , "Fly" , "Transmit music or video online" , "Small narrow flow" , "'Brook, runnel (6)'" .)