Outstanding note on a historical Italian town (5)
I believe the answer is:
ostia
'historical italian town' is the definition.
(ancient Roman harbour city)
'outstanding note on a' is the wordplay.
'outstanding' becomes 'os' (accounting abbreviation).
'note' becomes 'ti' (musical note as in do re mi fa so la ti).
'on' is a charade indicator (letters next to each other).
'os'+'ti'+'a'='OSTIA'
(Other definitions for ostia that I've seen before include "port in former times" , "ancient town" , "The old port of Rome" , "Roman port" , "town near Rome" .)