Leave many trees devastated! (6)
I believe the answer is:
desert
'leave' is the definition.
(desert can mean to leave or abandon)
'many trees devastated' is the wordplay.
'many' becomes 'd' (Roman numeral for 500).
'devastated' indicates anagramming the letters (devastate literally means destroy or lay waste to).
'trees' anagrammed gives 'esert'.
'd'+'esert'='DESERT'
(Other definitions for desert that I've seen before include "Arid area of land" , "Kalahari, e.g." , "Sahara, Gobi, etc" , "an area short of water" , "ditch" .)