Incapable of keeping an even keel? (5)

I believe the answer is:

tipsy

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'incapable' is the definition.
Both the definition and answer are adjectives. Maybe you can see a link between them that I don't see?

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for tipsy that I've seen before include "Slightly intoxicated" , "A bit drunk" , "Slightly drunk" , "1 over 8" , "Inebriated" .)

I've seen this clue in the Evening Standard.

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