Flow-controlling device (5)
I believe the answer is:
valve
'flow-controlling device' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for valve that I've seen before include "Part of brass instrument" , "Structure with a flap to insure one-way flow of fluid" , "Fluid regulator" , "Device for controlling flow of gas through pipe" , "Device for controlling the flow of a fluid in a pipe" .)