Children's author not entirely captivating a European city (5)
I believe the answer is:
'european city' is the definition.
(I've seen this before)
'children's author not entirely captivating a' is the wordplay.
'children's author' becomes 'milne' (AA Milne).
'not entirely' means to remove the last letter.
'captivating' is an insertion indicator (I've seen this in other clues).
'milne' with its last letter taken off is 'miln'.
'miln' enclosing 'a' is 'MILAN'.
(Other definitions for milan that I've seen before include "Northern Italian city" , "Italian city with La Scala opera house" , "Doorstep deliverer" , "Capital of Lombardy" , "European city" .)