Oregon pine in capital city, securely fixed, shortened (7,3)

I believe the answer is:

douglas fir

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'oregon pine in capital city' is the definition.
I can't tell whether this defines the answer.

'securely fixed shortened' is the wordplay.
'securely' becomes 'douglas' (I can't justify this - if you can you should believe this answer much more).
'fixed' becomes 'firm' (I've seen this in another clue).
'shortened' means to remove the last letter.
'firm' with its last letter taken off is 'fir'.
'douglas'+'fir'='DOUGLAS FIR'

Can you help me to learn more?

(Another definition for douglas fir that I've seen is " tree".)

I've seen this clue in The Telegraph.

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