Possibly unlit before (5)
I believe the answer is:
until
'before' is the definition.
(I've seen this before)
'possibly unlit' is the wordplay.
'possibly' indicates anagramming the letters.
'unlit' anagrammed gives 'UNTIL'.
(Other definitions for until that I've seen before include "Not beyond (hour)" , "Up to (a point in time)" , "Before a given time" , "As late as" , "Up to that moment" .)