Doctor joins boat for capital (6)
I believe the answer is:
moscow
'capital' is the definition.
(Moscow is an example)
'doctor joins boat' is the wordplay.
'doctor' becomes 'mo' (medical officer).
'joins' says to put letters next to each other.
'boat' becomes 'scow' (scow is a kind of boat).
'mo'+'scow'='MOSCOW'
'for' is the link.
(Other definitions for moscow that I've seen before include "Capital of Russia" , "Bolshoi Ballet base" , "Kremlin location" , "Russian city" , "City of 13" .)