Most are upset by distinguished musician (7)
I believe the answer is:
maestro
'distinguished musician' is the definition.
(I've seen this in another clue)
'most are upset' is the wordplay.
'upset' is an anagram indicator.
'most'+'are'='mostare'
'mostare' is an anagram of 'MAESTRO'.
'by' is the link.
(Other definitions for maestro that I've seen before include "Distinguished musician - most are (anag)" , "Expert in music etc" , "Gifted performer" , "ace" , "special performer" .)