Often got upset about king left behind (9)
I believe the answer is:
forgotten
'left behind' is the definition.
(I've seen this before)
'often got upset about king' is the wordplay.
'upset' indicates anagramming the letters.
'about' means one lot of letters goes inside another.
'king' becomes 'r' (r is abbreviation for rex).
'often'+'got'='oftengot'
'oftengot' anagrammed gives 'fogotten'.
'fogotten' placed around 'r' is 'FORGOTTEN'.
(Other definitions for forgotten that I've seen before include "Gone from memory" , "Unremembered" , "left behind" , "Not accessible to memory" , "No longer known" .)