Ominously, it appears the singer's suffering upset stomach (6)
I believe the answer is:
threat
'upset stomach' is the definition.
The definition and answer can be both states of affairs as well as being singular nouns.
Maybe they are linked in a way I don't understand?
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for threat that I've seen before include "Aggressive declaration" , "Warning of intent to harm" , "Statement of intent to do harm" , "Impending danger" , "Menacing words" .)