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