Trees not entirely original (4)
I believe the answer is:
firs
'trees' is the definition.
(fir is a kind of tree)
'not entirely original' is the wordplay.
'not entirely' means to remove the last letter (not all of the word).
'original' becomes 'first' (associated in meaning).
'first' with its last letter taken away is 'FIRS'.
(Other definitions for firs that I've seen before include "Evergreens" , "Trees" .)