Left in the dark (5)
I believe the answer is:
unlit
'left in the dark' is the definition.
(I've seen this before)
'left in the dark' is the wordplay.
'left' becomes 'l' (common abbreviation).
'in' is an insertion indicator.
'the dark' becomes 'unit' (I can't justify this - if you can you should believe this answer much more).
'l' put within 'unit' is 'UNLIT'.
Can you help me to learn more?
(Other definitions for unlit that I've seen before include "Not illuminated" , "not exactly bright" , "With no lighting" , "such as dark alley?" , "Not yet fired up" .)