Servant keeping doctor in capital (6)
I believe the answer is:
madrid
'capital' is the definition.
(capital of Spain)
'servant keeping doctor' is the wordplay.
'servant' becomes 'maid' (maid is a type of servant).
'keeping' is an insertion indicator (some letters kept inside).
'doctor' becomes 'dr' (abbreviation).
'maid' placed around 'dr' is 'MADRID'.
'in' is the link.
(Other definitions for madrid that I've seen before include "Spanish city" , "Spain's capital" , "European capital city" , "Spanish capital" , "Capital of Spain since 1561" .)