Fire extinguisher output (4)
I believe the answer is:
foam
'fire extinguisher output' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for foam that I've seen before include "Froth created by wave or soap" , "Suspension of gas in a liquid" , "Froth or spume" , "Suds" , "Mass of small bubbles" .)