Earliest trees close to forest (5)
I believe the answer is:
first
'earliest' is the definition.
(I've seen this before)
'trees close to forest' is the wordplay.
'trees' becomes 'firs' (fir is a kind of tree).
'close to' indicates one should take the final letters (the close/ending of).
The last letter of 'forest' is 't'.
'firs'+'t'='FIRST'
(Other definitions for first that I've seen before include "Preceding all others" , "Premier (except in football!)" , "Foremost" , "higher than the others" , "Position for gold" .)