It's glee that's upset a writer of sad poems (7)
I believe the answer is:
elegist
'a writer of sad poems' is the definition.
(I've seen this in another clue)
'it's glee that's upset' is the wordplay.
'that's upset' indicates an anagram.
'its'+'glee'='itsglee'
'itsglee' is an anagram of 'ELEGIST'.
(Other definitions for elegist that I've seen before include "Composer of mournful songs" , "Kind of writer" , "serious linesman" , "Gloomy poet" , "one wrote mournfully" .)