Earliest tree on street (5)
I believe the answer is:
first
'earliest' is the definition.
(I've seen this before)
'tree on street' is the wordplay.
'tree' becomes 'fir' (fir is a kind of tree).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'street' becomes 'st' (abbreviation).
'fir'+'st'='FIRST'
(Other definitions for first that I've seen before include "Highest degree" , "Cardinal" , "Preliminary" , "Foremost" , "at the outset" .)