Dullness of work by an urban area (7)
I believe the answer is:
opacity
'dullness' is the definition.
(thesaurus)
'work by an urban area' is the wordplay.
'work' becomes 'op' (abbreviation for opus).
'by' says to put letters next to each other.
'an' becomes 'a'.
'urban area' becomes 'city' (city is a kind of urban area).
'op'+'a'+'city'='OPACITY'
'of' is the link.
(Other definitions for opacity that I've seen before include "Lack of transparency" , "Not allowing passage through" , "Obscureness" , "Unclear quality" , "incomprehensible" .)