Subject of cautionary tale sorrowful, I am upset (5)
I believe the answer is:
midas
'subject of cautionary tale' is the definition.
(as in the mythical King Midas)
'sorrowful i am upset' is the wordplay.
'sorrowful' becomes 'sad' (similar in meaning).
'i am' becomes 'I'm' (contraction).
'upset' says the letters should be written in reverse.
'sad'+'im'='sadim'
'sadim' back-to-front is 'MIDAS'.
(Other definitions for midas that I've seen before include "Greek mythological king with money-making touch" , "old king" , "my gift turned out a curse" , "Mythical king with golden touch" , "Gold-creating king" .)