Environmental case cited by upset socialist refers to tree (5)
I believe the answer is:
elder
'tree' is the definition.
(elder is a kind of tree)
'environmental case cited by upset' is the wordplay.
'case' means to remove the middle letters.
'cited by' becomes 'red' (I am not sure about this - if you are sure you should believe this answer much more).
'upset' shows that the letters should be reversed in order.
'environmental' with its middle removed is 'el'.
'red' written backwards gives 'der'.
'el'+'der'='ELDER'
'socialist refers to' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for elder that I've seen before include "one with roots" , "Layman of the church" , "Senior - church official" , "Church leader; tree" , "Tree and Mormon church officer" .)