Not going under with a petty cash sum (6)

I believe the answer is:

afloat

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'not going under' is the definition.
(I've seen this in another clue)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for afloat that I've seen before include "At sea - in circulation" , "Bobbing along?" , "With one's head above water" , "Still solvent" , "Borne on the water" .)

I've seen this clue in The Sun.

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