Fever starts to fluctuate (3)
I believe the answer is:
flu
'fever' is the definition.
The definition and answer can be both states of affairs as well as being singular nouns.
Perhaps there's an association between them I don't understand?
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for flu that I've seen before include "In brief, winter illness" , "Swine --; gastric --" , "Common ailment" , "A bad winter bug" , "Common annual illness" .)