Crossword Genius

Trees planted in middle of Seattle initially (2,5)

Ross

I believe the answer is:

at first

I'm a little stuck... Click here to teach me more about this clue!

'initially' is the definition.
(synonyms)

'trees planted in middle of seattle' is the wordplay.
I cannot really understand how this works, but
'trees' could be 'firs' (fir is a kind of tree) and 'firs' is found in the answer.
The remaining letters 'att' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for at first that I've seen before include "In the beginning, initially" , "To begin with" , "Originally" , "In the beginning, to start with" .)

I've seen this clue in The Telegraph.
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