I'd seem upset by a death (6)
I believe the answer is:
demise
'a death' is the definition.
('demise' can be a synonym of 'death')
'i'd seem upset' is the wordplay.
'upset' indicates an anagram.
'id'+'seem'='idseem'
'idseem' anagrammed gives 'DEMISE'.
'by' is the link.
(Other definitions for demise that I've seen before include "(Person's) death" , "Passing" , "The end of something" , "Death or expiration" , "Death, collapse" .)