Unconventional filler used in heraldic device (5-2-3)

I believe the answer is:

fleur-de-lis

'heraldic device' is the definition.

'unconventional filler used' is the wordplay.
'unconventional' indicates anagramming the letters.
'filler'+'used'='fillerused'
'fillerused' is an anagram of 'FLEUR-DE-LIS'.

'in' is the link.

I've seen this clue in The Guardian.

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