Unilluminated section around entrance of lobby (5)
I believe the answer is:
unlit
'unilluminated' is the definition.
(synonyms)
'section around entrance of lobby' is the wordplay.
'section' becomes 'unit' (I've seen this before).
'around' means one lot of letters goes inside another.
'entrance of' indicates taking the first letters.
The initial letter of 'lobby' is 'l'.
'unit' placed around 'l' is 'UNLIT'.
(Other definitions for unlit that I've seen before include "Not yet fired up" , "Perhaps invisible" , "Dark, gloomy" , "not provided with a lamp?" , "Without illumination" .)