Protecting Australia, Darwin shall reassemble defensive structure (8,4)

I believe the answer is:

hadrian's wall

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'defensive structure' is the definition.
Both the answer and definition are singular nouns.
Perhaps there's a link between them I don't understand?

'protecting australia darwin shall reassemble' is the wordplay.
'protecting' is an insertion indicator (inserted letters are held or protected).
'australia' becomes 'A' (abbreviation for Australia).
'reassemble' is an anagram indicator (I've seen 'reassembling' mean this).
'darwin'+'shall'='darwinshall'
'darwinshall' with letters rearranged gives 'hadrinswall'.
'a' placed into 'hadrinswall' is 'HADRIANS WALL'.

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I've seen this clue in The Everyman puzzle in The Observer.

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