Trees planted in middle of Seattle initially (2,5)
I believe the answer is:
at first
'initially' is the definition.
(synonyms)
'trees planted in middle of seattle' is the wordplay.
'trees' becomes 'firs' (fir is a kind of tree).
'planted in' means one lot of letters goes inside another.
'middle of' means to look at the middle letters.
The central letters of 'seattle' are 'att'.
'firs' going inside 'att' is 'AT FIRST'.
(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" .)