Master switch makes it flow (6)
I believe the answer is:
stream
'flow' is the definition.
(I know that flow can be written as stream)
'master switch' is the wordplay.
'switch' indicates anagramming the letters.
'master' is an anagram of 'STREAM'.
'makes it' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for stream that I've seen before include "divide pupils into ability groups" , "River - current" , "selected pupils" , "Transmit music or video online" , "comprising a load of drips?" .)