Sap in scarlet went to bed (7)
I believe the answer is:
retired
'went to bed' is the definition.
('retire' can be a synonym of 'go to bed')
'sap in scarlet' is the wordplay.
'sap' becomes 'tire' (tiring is a kind of sapping).
'in' indicates putting letters inside.
'scarlet' becomes 'red' (synonyms).
'tire' put into 'red' is 'RETIRED'.
(Other definitions for retired that I've seen before include "went back" , "Went to bed or gave up work" , "Turned in" , "Withdrew from active participation" , "Withdrawn from active participation" .)