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 wordplay.
'upset' indicates an anagram.
'doendflu' is an anagram of 'UNFOLDED'.
'that's' is the link.
(Other definitions for unfolded that I've seen before include "developed" and "Revealed; spread out".)