Demand navy to attend old Italian port (7)
I believe the answer is:
salerno
'italian port' is the definition.
(port city in Italy)
'demand navy to attend old' is the wordplay.
'demand' becomes 'sale' (as in supply and demand).
'navy' becomes 'RN' (abbreviation for Royal Navy).
'to attend' means one lot of letters go next to another.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'sale'+'rn'+'o'='SALERNO'
(Other definitions for salerno that I've seen before include "European city!" , "Campanian port" , "Italian city; Orleans (anag.)" , "Battle" , "South Italian port" .)