Food shop in abandoned citadel to north of Ruhr city (12)
I believe the answer is:
delicatessen
'food shop' is the definition.
('delicatessen' can be a synonym of 'food shop')
'abandoned citadel to north of ruhr city' is the wordplay.
'abandoned' is an anagram indicator.
'to north of' says to put letters next to each other (in a down clue, some letters go above others).
'ruhr city' becomes 'essen' (I've seen this before).
'citadel' anagrammed gives 'delicat'.
'delicat'+'essen'='DELICATESSEN'
'in' is the link.
(Other definitions for delicatessen that I've seen before include "Here one gets exotic foods" , "food store?" , "Specialised food shop" , "where to get fare from abroad" , "Shop selling exotic prepared foods" .)