Rich Green upset a lot of aldermen (7)
I believe the answer is:
emerald
'rich green' is the definition.
(I've seen this in another clue)
'upset a lot of aldermen' is the wordplay.
'upset' indicates an anagram.
'a lot of' means to remove the last letter (most of the word but not all of it).
'aldermen' with its final letter removed is 'alderme'.
'alderme' with letters rearranged gives 'EMERALD'.
(Other definitions for emerald that I've seen before include "On of the Greens" , "...re medal? (Anag)" , "Highly-valued variety of beryl" , "Shade of green" , "Green gem" .)