Maybe keep undermining modern city (9)
I believe the answer is:
newcastle
'city' is the definition.
(Newcastle is an example)
'maybe keep undermining modern' is the wordplay.
'maybe keep' becomes 'castle' (I've seen this before. I am not sure about the 'maybe' bit.).
'undermining' means one lot of letters go next to another.
'modern' becomes 'new' (synonyms).
'castle' after 'new' is 'NEWCASTLE'.
Can you help me to learn more?
(Other definitions for newcastle that I've seen before include "town" , "port for England" , "Man City?" , "City on the Tyne" , "English city" .)