Returning more ashen, starts to suffer extreme setback (7)
I believe the answer is:
relapse
'setback' is the definition.
(I've seen this before)
'returning more ashen starts to suffer extreme' is the wordplay.
'returning' says the letters should be written backwards.
'more ashen' becomes 'paler' ('pale' can be a synonym of 'ashen').
'starts to' says to take the initial letters.
The initial letters of 'suffer extreme' is 'se'.
'paler' written backwards gives 'relap'.
'relap'+'se'='RELAPSE'
(Other definitions for relapse that I've seen before include "Take a turn for the worse" , "Decline" , "Failure to maintain a higher state" , "medical problem" , "Slip back to former state" .)