Fall back into bad health (7)
I believe the answer is:
relapse
'fall back into bad health' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for relapse that I've seen before include "Regress, fall back" , "Decline" , "Deterioration after period of improvement" , "Fall back into illness" , "Slip back to former state" .)