Single covering remains dirty (8)
I believe the answer is:
unwashed
'dirty' is the definition.
('unwashed' can be similar in meaning to 'dirty')
'single covering remains' is the wordplay.
'single' becomes 'unwed' ('unwed' can be a synonym of 'single').
'covering' indicates putting letters inside.
'remains' becomes 'ash' (I've seen this before).
'unwed' enclosing 'ash' is 'UNWASHED'.
(Other definitions for unwashed that I've seen before include "they were great once!" , "Dirty" .)