Green alder decaying below upset me (7)
I believe the answer is:
emerald
'green' is the definition.
(I know that emerald is a type of green)
'alder decaying below upset me' is the wordplay.
'decaying' is an anagram indicator (the original order breaks down).
'below' means one lot of letters go next to another (in a down clue, some letters go below others).
'upset' says the letters should be written backwards.
'me' written backwards gives 'em'.
'alder' with letters rearranged gives 'erald'.
'erald' put after 'em' is 'EMERALD'.
(Other definitions for emerald that I've seen before include "Precious gemstone" , "Mere lad (anag.)" , "Green like this isle" , "Precious green stone" , "Gem; type of moth" .)