He's employed because assorted trees are required (8)
I believe the answer is:
forester
'he's employed' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'because assorted trees are required' is the wordplay.
'because' becomes 'for' (I've seen this in other clues).
'assorted' is an anagram indicator.
'are required' is a charade indicator (letters next to each other).
'trees' anagrammed gives 'ester'.
'for'+'ester'='FORESTER'
Can you help me to learn more?
(Other definitions for forester that I've seen before include "One works among trees" , "One caring for trees" , "Woodman" , "One in charge of woodlands" , "Tree expert" .)