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' indicates taking the first letters.
The initial letters of 'suffer extreme' is 'se'.
'paler' backwards is '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" .)