Artist I’d upset about almost everything (4)
I believe the answer is:
dali
'artist' is the definition.
(I've seen this before)
'i'd upset about almost everything' is the wordplay.
'upset' indicates an anagram.
'about' is an insertion indicator.
'almost' means to remove the last letter (most of the word but not all of it).
'everything' becomes 'all' (I've seen this before).
'id' anagrammed gives 'di'.
'all' with its final letter taken away is 'al'.
'di' enclosing 'al' is 'DALI'.
(Other definitions for dali that I've seen before include "Tree" , "Salvador . . . . was great Spanish surrealist painter" , "Salvador --, twentieth century Spanish surrealist painter" , "Salvador -, twentieth century artist" , "Controversial painter" .)