Sign of resistance in Spanish port (5)
I believe the answer is:
virgo
'sign' is the definition.
(I know that Virgo is a sign)
'resistance in spanish port' is the wordplay.
'resistance' becomes 'r' (electronics abbreviation).
'in' indicates putting letters inside.
'spanish port' becomes 'Vigo' (port in Spain).
'r' placed inside 'vigo' is 'VIRGO'.
'of' acts as a link.
(Other definitions for virgo that I've seen before include "Large constellation between Leo and Libra" , "Star sign" , "Maiden" , "House" , "Sixth sign of zodiac" .)