How one might defend being devious about a ticket seller (7)

I believe the answer is:

stoutly

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

'how one might defend' is the definition.
I can't tell whether this defines the answer.

'devious about a ticket seller' is the wordplay.
'devious' becomes 'sly' (I've seen this before).
'about' is an insertion indicator.
'a ticket seller' becomes 'tout' (I've seen this before).
'sly' going around 'tout' is 'STOUTLY'.

'being' acts as a link.

Can you help me to learn more?

(Another definition for stoutly that I've seen is " Bravely".)

I've seen this clue in the Evening Standard.

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