Catch, all but overturning the boat (4)

I believe the answer is:

grab

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'catch' is the definition.
(I know that catch can be written as grab)

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for grab that I've seen before include "Seize roughly" , "Brag about seizure" , "Seize hold of" , "Seize suddenly" , "Impress" .)

I've seen this clue in the Evening Standard.

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