Urban underground river illuminated by article (12)
I believe the answer is:
metropolitan
'urban' is the definition.
(I've seen this before)
'underground river illuminated by article' is the wordplay.
'underground' becomes 'metro' (synonyms).
'river' becomes 'Po' (Italian river).
'illuminated' becomes 'lit' (synonyms).
'by article' becomes 'an' (English indefinite article. I am not sure about the 'by' bit.).
'metro'+'po'+'lit'+'an'='METROPOLITAN'
Can you help me to learn more?
(Other definitions for metropolitan that I've seen before include "Of a large city -- London Underground line" , "Appertaining to a big city ..." , "Urban" , "senior bishop?" , "religious leader" .)