Son beginning to like wearing glasses in capital (4)
I believe the answer is:
oslo
'capital' is the definition.
(capital of Norway)
'son beginning to like wearing glasses' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'beginning to' suggests taking the first letters.
'wearing' is an insertion indicator.
'glasses' becomes 'OO' (OO looks like spectacles).
The initial letter of 'like' is 'l'.
's'+'l'='sl'
'sl' inserted inside 'oo' is 'OSLO'.
'in' is the link.
(Other definitions for oslo that I've seen before include "Northern capital of Czechoslovakia?" , "European capital in go-slow" , "Capital in throes of go-slow" , "Former Christiania" , "Solo in Norway?" .)