Working until dark (5)
I believe the answer is:
unlit
'dark' is the definition.
(similar in meaning)
'working until' is the wordplay.
'working' indicates anagramming the letters.
'until' anagrammed gives 'UNLIT'.
(Other definitions for unlit that I've seen before include "not exactly bright" , "Left in the dark" , "Not illuminated" , "not provided with a lamp?" , "Not yet burning" .)