Regressed (8)
I believe the answer is:
relapsed
'regressed' is the definition.
('relapse' can be a synonym of 'regress')
This is the entire clue.
(Other definitions for relapsed that I've seen before include "Weakened" , "Regressed, got worse" , "Suffered setback" , "having become ill again" , "Returned to a poorer state of health" .)