Work on a big town in darkness (7)
I believe the answer is:
opacity
'darkness' is the definition.
(thesaurus)
'work on a big town' is the wordplay.
'work' becomes 'op' (common abbreviation for 'opus').
'on' is a charade indicator (letters next to each other).
'big town' becomes 'city' (I've seen this before).
'op'+'a'+'city'='OPACITY'
'in' acts as a link.
(Other definitions for opacity that I've seen before include "Non-transparency" , "incomprehensible" , "Not allowing passage through" , "Unclear quality" , "Cloudiness" .)