In Spanish port, a Republican sign (5)
I believe the answer is:
virgo
'sign' is the definition.
(I know that Virgo is a sign)
'in spanish port a republican' is the wordplay.
'in' indicates putting letters inside.
'spanish port' becomes 'Vigo' (port in Spain).
'a republican' becomes 'r' (political abbreviation).
'vigo' placed around 'r' is 'VIRGO'.
(Other definitions for virgo that I've seen before include "number of stars" , "Large constellation between Leo and Libra" , "Large constellation -- sign of the zodiac" , "Zodiacal sign" , "Maiden" .)