They are dead fasteners for a barrier (9)

I believe the answer is:

doornails

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'they are dead' is the definition.
The definition suggests an adverb but the answer is not.

'fasteners for a barrier' is the wordplay.
'fasteners' becomes 'nails' (nail is a kind of fastener).
'for' says to put letters next to each other (I've seen this in other clues).
'a barrier' becomes 'door' (door is a kind of barrier).
'nails' put after 'door' is 'DOORNAILS'.

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I've seen this clue in the Sydney Morning Herald.

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