Crossword Genius

As a finish to hatter's ambition? (2,3,2,3)

Ross

I believe the answer is:

to cap it all

I'm a little stuck... Click here to teach me more about this clue!

'as a finish' is the definition.
I know nothing about this answer so I cannot judge whether this works.

'to hatter's ambition?' is the wordplay.
I cannot quite see how this works, but
'to' is present in the answer.
The remaining letters 'capitall' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for to cap it all that I've seen before include "On top of everything else" , "To provide the finishing touch" , "Finally" , "As a (frequently unwanted) climax" , "as a last straw" .)

I've seen this clue in The Guardian.
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