Firmly cunning about squabble (7)

I believe the answer is:

stiffly

'firmly' is the definition.
(thesaurus)

'cunning about squabble' is the wordplay.
'cunning' becomes 'sly' (synonyms).
'about' indicates putting letters inside.
'squabble' becomes 'tiff' (synonyms).
'sly' placed around 'tiff' is 'STIFFLY'.

(Another definition for stiffly that I've seen is " With an rigid manner".)

I've seen this clue in The Irish Independent.

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