Bombast is old Falstaff’s hallmark (10)

I believe the answer is:

orotundity

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'bombast' is the definition.
(thesaurus)

'old falstaff's hallmark' is the wordplay.
I cannot really see how this works, but
'old' could be 'o' (common abbreviation eg in OE for Old English) and 'o' is found within the answer.
The remaining letters 'rotundity' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...

'is' acts as a link.

Can you help me to learn more?

I've seen this clue in The Times.

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