Dark one gets left in (5)
I believe the answer is:
unlit
'dark' is the definition.
(similar in meaning)
'one gets left in' is the wordplay.
'one' becomes 'un' (northern English dialect form of 'one').
'gets left in' becomes 'lit' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'un'+'lit'='UNLIT'
Can you help me to learn more?
(Other definitions for unlit that I've seen before include "Not yet fired up" , "such as dark alley?" , "Not illuminated" , "Not yet burning" , "not exactly bright" .)