Fell back in rush around the circuits (8)
I believe the answer is:
relapsed
'fell back' is the definition.
('relapse' can be a synonym of 'fall back')
'rush around the circuits' is the wordplay.
'rush' becomes 'reed' (I've seen this before).
'around' indicates putting letters inside.
'the circuits' becomes 'laps' ('lap' can be a synonym of 'circuit').
'reed' enclosing 'laps' is 'RELAPSED'.
'in' is the link.
(Other definitions for relapsed that I've seen before include "Fell back (into sickness)" , "Dropped back" , "Regressed, got worse" , "Weakened" , "Returned to a poorer state of health" .)