Trees not found in the mountains, we’re told (6)
I believe the answer is:
planes
'trees not' is the definition.
'planes' can be an answer for 'trees' (I've seen this before). I am not certain of the 'not' bit.
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for planes that I've seen before include "Those flying" , "Aircraft and tools for smoothing wood" , "Jets maybe" , "Aircraft from Naples" , "Flat surface in Naples" .)