Turning paler, initially suffer extreme return to bad health (7)
I believe the answer is:
relapse
'return to bad health' is the definition.
'turning paler initially suffer extreme' is the wordplay.
'turning' says the letters should be written backwards.
'initially' indicates taking the first letters.
The first letters of 'suffer extreme' is 'se'.
'paler' back-to-front is 'relap'.
'relap'+'se'='RELAPSE'
(Other definitions for relapse that I've seen before include "Downturn in health (after improvement)" , "Become ill again" , "Fall back into bad health" , "Backsliding" , "Take a turn for the worse" .)