Trees seen in middle of Seattle initially (2,5)
I believe the answer is:
at first
'initially' is the definition.
(synonyms)
'trees seen in middle of seattle' is the wordplay.
'trees' becomes 'firs' (fir is a kind of tree).
'seen in' means one lot of letters goes inside another.
'middle of' indicates the central letters.
The centre of 'seattle' is 'att'.
'firs' inserted into 'att' is 'AT FIRST'.
(Other definitions for at first that I've seen before include "Originally" , "To begin with" , "In the beginning, to start with" , "In the beginning, initially" .)