I'll say that's an Arabian vessel (3,3)

I believe the answer is:

and how

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'i'll' is the definition.
I can't tell whether this defines the answer.

'that's an arabian vessel' is the wordplay.
'that's an' becomes 'an'.
'arabian vessel' becomes 'dhow' (I've seen this in another clue).
'an'+'dhow'='AND HOW!'

'say' acts as a link.

Can you help me to learn more?

(Another definition for and how that I've seen is " Certainly".)

I've seen this clue in The Times.

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