Control torrential stream (6)
I believe the answer is:
master
'control' is the definition.
(I know that control can be written as master)
'torrential stream' is the wordplay.
'torrential' indicates an anagram (letters in a storm or torrent).
'stream' is an anagram of 'MASTER'.
(Other definitions for master that I've seen before include "Virtuoso" , "Schoolteacher" , "An old one is one of the early and great European painters" , "Overcome; original copy" , "Overcome; original film" .)