It thus set up a Roman port (5)
I believe the answer is:
ostia
'roman port' is the definition.
(I've seen this before)
'it thus set up a' is the wordplay.
'thus' becomes 'so' (synonyms).
'set up' shows that the letters should be reversed in order.
'it'+'so'='itso'
'itso' backwards is 'osti'.
'osti'+'a'='OSTIA'
(Other definitions for ostia that I've seen before include "openings in Rome" , "Ancient harbour" , "port in former times" , "Small openings" , "ruined port" .)