Do end flu upset that's developed (8)
I believe the answer is:
unfolded
'developed' is the definition.
(unfolding is a kind of developing)
'do end flu upset' is the wordplay.
'upset' indicates an anagram.
'do'+'end'+'flu'='doendflu'
'doendflu' is an anagram of 'UNFOLDED'.
'that's' is the link.
(Other definitions for unfolded that I've seen before include "developed" , "Revealed; spread out" .)