Crossword Genius

What a Hoover does when it works (and doesn't) (5)

Ross

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" .)

I've seen this clue in The Guardian.
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