Dark one has left (5)
I believe the answer is:
unlit
'dark' is the definition.
(similar in meaning)
'one has left' is the wordplay.
'one' becomes 'unit' (synonyms).
'has' is an insertion indicator.
'left' becomes 'l' (common abbreviation).
'unit' placed around 'l' is 'UNLIT'.
(Other definitions for unlit that I've seen before include "Perhaps invisible" , "Not yet fired up" , "Not illuminated" , "Without illumination" , "such as dark alley?" .)