Retreats within borders of one European city (6)
I believe the answer is:
odense
'european city' is the definition.
'odense' can be an answer for 'city' (I have seen 'Danish city' mean 'odense' so perhaps 'city' could also mean 'odense'). I am not certain of the 'european' bit.
'retreats within borders of one' is the wordplay.
'retreats' becomes 'dens' (I've seen this in another clue).
'within' is an insertion indicator.
'borders of' says to hollow out the word (remove centre letters).
'one' with its centre removed is 'oe'.
'dens' going inside 'oe' is 'ODENSE'.
Can you help me to learn more?
(Other definitions for odense that I've seen before include "foreign port?" , "Danish city" , "in Denmark?" , "where Canute founded a cathedral" , "Foreign city" .)