Good artist endlessly upset for nothing (6)
I believe the answer is:
gratis
'nothing' is the definition.
(I've seen this before)
'good artist endlessly upset' is the wordplay.
'good' becomes 'g' (abbreviation).
'endlessly' means to remove the last letter.
'upset' indicates an anagram.
'artist' with its last letter taken off is 'artis'.
'artis' with letters rearranged gives 'ratis'.
'g'+'ratis'='GRATIS'
'for' acts as a link.
(Other definitions for gratis that I've seen before include "Free (Latin)" , "Without charge" , "On the house" , "Costing nothing, free" , "Free of charge" .)