Daughter learning about the St Petersburg of old (9)
I believe the answer is:
leningrad
'st petersburg of old' is the definition.
(St Petersburg was previously called Leningrad)
'daughter learning about' is the wordplay.
'daughter' becomes 'd' (genealogical abbreviation).
'about' indicates anagramming the letters.
'd'+'learning'='dlearning'
'dlearning' is an anagram of 'LENINGRAD'.
'the' is the link.
(Other definitions for leningrad that I've seen before include "St Petersburg before nineteen ninety one" , "Name of St Petersburg from 1924 to 1991" , "St Petersburg some time ago" , "Former name of city" , "Soviet period name for St Petersburg" .)