Not just teeth wobble about when drinking litres (5,3,4)

I believe the answer is:

below the belt

'not just' is the definition.

I can't explain the rest of the clue.

(Other definitions for below the belt that I've seen before include "position of trousers" , "Underhand, unfair" , "out of order" , "Unfair, disregarding the rules" , "dirty" .)

I've seen this clue in The Guardian.

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