Clean sweep - tricky! (4)

I believe the answer is:

slam

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'clean sweep' is the definition.
'slam' can be an answer for 'sweep' (synonyms). I'm not sure about the 'clean' bit.

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for slam that I've seen before include "Close noisily" , "ambitious bid" , "Criticise - attack" , "Shut (door) with force" , "Winning all (or all but one) of the tricks in bridge" .)

I've seen this clue in The Irish Independent.

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