Place on Italian coast with very big island to the west (5)
I believe the answer is:
ostia
'place on italian coast' is the definition.
(Italian town)
'with very big island to the west' is the wordplay.
'with' is a charade indicator (letters next to each other).
'very big' becomes 'os' (abbreviation for outsize).
'island' becomes 'ait' (term for a small island).
'to the west' says the letters should be written in reverse (letters written right to left or \'east to west\').
'ait' written backwards gives 'tia'.
'os' put next to 'tia' is 'OSTIA'.
(Other definitions for ostia that I've seen before include "town near Rome" , "ruined port" , "Roman port" , "port in former times" , "openings in Rome" .)