What a Hoover does when it works (and doesn't) (5)
I believe the answer is:
sucks
'what a hoover does when it works' is the definition.
(vacuum cleaners suck up dirt)
I cannot understand how the remainder of the clue works.
(Other definitions for sucks that I've seen before include "Makes a partial vacuum with the mouth" , "Uses a straw" , "Draws into the mouth, liquid or air" .)