One's left in dark (5)
I believe the answer is:
unlit
'dark' is the definition.
('unlit' can be similar in meaning to 'dark')
'one's left in' is the wordplay.
'one' becomes 'unit' ('unit' can be a synonym of 'one').
'left' becomes 'l' (common abbreviation).
'in' means one lot of letters goes inside another.
'unit' enclosing 'l' is 'UNLIT'.
(Other definitions for unlit that I've seen before include "Left in the dark" , "Shadowy" , "not exactly bright" , "Not illuminated" , "Lacking illumination" .)