Runs circuits in case of extreme deterioration in health (7)
I believe the answer is:
relapse
'deterioration in health' is the definition.
(I've seen this before)
'runs circuits in case of extreme' is the wordplay.
'runs' becomes 'r' (cricket abbreviation).
'circuits' becomes 'laps' ('lap' can be a synonym of 'circuit').
'in' is an insertion indicator.
'case of' suggests removing the centre (outsides of).
'extreme' with its middle removed is 'ee'.
'laps' put into 'ee' is 'elapse'.
'r'+'elapse'='RELAPSE'
(Other definitions for relapse that I've seen before include "Repeals (anag.)" , "A return to illness" , "fall into sin" , "Deteriorate after getting better" , "Medical setback" .)