Timber cross we'd erroneously inscribed (7)

I believe the answer is:

redwood

'timber' is the definition.

I don't understand how the rest of the clue works.

(Other definitions for redwood that I've seen before include "John -, MP; large tree" , "Giant North American conifer" , "Mad" , "Tory MP" , "former Tory minister" .)

I've seen this clue in The Times.

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