Miserable? I'm about to become a king (5)
I believe the answer is:
midas
'a king' is the definition.
(mythical king)
'miserable? i'm about' is the wordplay.
'miserable?' becomes 'sad' ('sad' can be similar in meaning to 'miserable').
'about' says the letters should be written backwards.
'sad'+'im'='sadim'
'sadim' written backwards gives 'MIDAS'.
'to become' is the link.
(Other definitions for midas that I've seen before include "King of Phrygia who had a golden touch" , "Golden-touch king" , "Legendary king with golden touch" , "Mythical king with golden touch" , "my gift turned out a curse" .)