City diary penned by a toff from the south (7)
I believe the answer is:
bologna
'city' is the definition.
(city in Italy)
'diary penned by a toff from the south' is the wordplay.
'diary' becomes 'log' (log can mean a record or diary).
'penned by' indicates putting letters inside (inserted letters are penned in).
'toff' becomes 'nob' (UK slang for a wealthy or upper-class person).
'from the south' says the letters should be written in reverse (in a down clue, letters go up).
'a'+'nob'='anob'
'anob' written backwards gives 'bona'.
'log' placed into 'bona' is 'BOLOGNA'.
(Other definitions for bologna that I've seen before include "Italian province" , "Capital of Emilia-Romagna" , "Italian city - smoked sausage" , "North Italian university city" .)