Positively declare contrivance saves a tree (10)

I believe the answer is:

asseverate

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'positively declare' is the definition.
'asseverate' can be an answer for 'declare' (I've seen this before). I'm not certain of the 'positively' bit.

'contrivance saves a tree' is the wordplay.
'contrivance' indicates an anagram.
'saves'+'a'+'tree'='savesatree'
'savesatree' anagrammed gives 'ASSEVERATE'.

Can you help me to learn more?

(Other definitions for asseverate that I've seen before include "State categorically" , "Swear" , "Claim" , "Solemnly say" , "Solemnly declare" .)

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

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