Do end flu upset that's developed (8)
I believe the answer is:
Here is my best explanation:
'developed' is the definition.
(unfolding is a kind of developing)
'do end flu upset' is the subsidiary indication.
'upset' indicates an anagram.
'doendflu' anagrammed gives 'unfolded'.
'that's' is the link.
(Another definition for unfolded that I've seen is "Revealed; spread out".)