Old Italian family provides information on doctor (6)
I believe the answer is:
medici
'old italian family' is the definition.
(I've seen this before)
'information on doctor' is the wordplay.
'information' becomes 'i' (common abbreviation for information).
'on' means one lot of letters go next to another.
'doctor' becomes 'medic' (doctor is a type of medic).
'i' put after 'medic' is 'MEDICI'.
'provides' acts as a link.
(Other definitions for medici that I've seen before include "Florentine ruling family" , "Ruling family of Renaissance Florence" , "Noted Florentine family" , "Powerful Italian family in the 15th century" , "Name of great Florence family in 15-17 centuries" .)