Doctor one leads to Italian family (6)
I believe the answer is:
medici
'italian family' is the definition.
(I've seen this before)
'doctor one' is the wordplay.
'doctor' becomes 'medic' (doctor is a type of medic).
'one' becomes 'i' (Roman numeral).
'medic'+'i'='MEDICI'
'leads to' acts as a link.
(Other definitions for medici that I've seen before include "Name of great Florentine family 15-17th centuries" , "Prominent family of Florence" , "Great Italian family" , "Noted Florentine family" , "Noble Florentine family" .)