Castle smithy, Pym, ruined extensive body network (9,6)

I believe the answer is:

lymphatic system

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'extensive body network' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.

'castle smithy pym ruined' is the wordplay.
'ruined' indicates anagramming the letters.
'castle'+'smithy'+'pym'='castlesmithypym'
'castlesmithypym' is an anagram of 'LYMPHATIC SYSTEM'.

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

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