Reverted (8)
(Other definitions for relapsed that I've seen before include "Sank back (into illness)" , "Deteriorated in health again" , "Failed to maintain improvement" , "Dropped back" , "Weakened" .)
'reverted' is the definition.
('return' can be a synonym of 'revert')
This is the entire clue.
(Other definitions for returned that I've seen before include "dishonoured by the bank" , "Honoured by the electorate" , "Sent back" , "Went, came or gave back" , "Went, came or brought back" .)