Son in Holy Writ originally inhabiting a Roman port (5)
I believe the answer is:
ostia
'roman port' is the definition.
(ancient Roman port)
'son in holy writ originally inhabiting a' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'in' indicates putting letters inside.
'holy writ' becomes 'OT' (Old Testament).
'originally' says to take the initial letters.
The first letter of 'inhabiting' is 'i'.
's' placed into 'ot' is 'ost'.
'ost'+'i'+'a'='OSTIA'
(Other definitions for ostia that I've seen before include "ruined port" , "port in former times" , "Roman port" , "The old port of Rome" , "town near Rome" .)