Fashionable young woman, Italian, linked with doctor turned fifty (2,4)
I believe the answer is:
it girl
'fashionable young woman' is the definition.
(I know that it girl is a subclass of fashionable fille)
'italian linked with doctor turned fifty' is the wordplay.
'italian' becomes 'it' (abbreviation for Italian).
'linked with' says to put letters next to each other.
'doctor' becomes 'rig' (both can mean to manipulate or tamper with).
'turned' says the letters should be written in reverse.
'fifty' becomes 'l' (Roman numeral).
'rig' back-to-front is 'gir'.
'it'+'gir'+'l'='IT GIRL'
(Other definitions for it girl that I've seen before include "Highly-attractive young woman" , "Socialite" , "lass with lots of oomph" .)