Dark when light's first penetrating apartment (5)
I believe the answer is:
unlit
'dark' is the definition.
(similar in meaning)
'light's first penetrating apartment' is the wordplay.
'light's first' becomes 'l' (1st letter of 'light').
'penetrating' indicates putting letters inside.
'apartment' becomes 'unit' (I've seen this in another clue).
'l' placed inside 'unit' is 'UNLIT'.
'when' acts as a link.
(Other definitions for unlit that I've seen before include "Without illumination" , "not provided with a lamp?" , "Until (anag.)" , "Perhaps invisible" , "Not illuminated" .)