French one left it dark (5)
I believe the answer is:
unlit
'dark' is the definition.
(similar in meaning)
'french one left it' is the wordplay.
'french one' becomes 'un' ('one' in French).
'left' becomes 'l' (common abbreviation).
'un'+'l'+'it'='UNLIT'
(Other definitions for unlit that I've seen before include "not exactly bright" , "Lacking illumination" , "Left in the dark" , "not provided with a lamp?" , "such as dark alley?" .)