Prime trees doomed (5,2,3)

I believe the answer is:

first of all

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'prime' is the definition.
(I've seen this before)

'trees doomed' is the wordplay.
'trees' becomes 'firs' (fir is a kind of tree).
'doomed' becomes 'tofall' (I am not sure about this - if you are sure you should believe this answer much more).
'firs'+'tofall'='FIRST OF ALL'

Can you help me to learn more?

(Other definitions for first of all that I've seen before include "initially" , "Prime" , "Before anything else" .)

I've seen this clue in The Globe and Mail.

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