Portia terribly tense as nationalist (7)
I believe the answer is:
patriot
'nationalist' is the definition.
(synonyms)
'portia terribly tense' is the wordplay.
'terribly' is an anagram indicator.
'tense' becomes 't' (abbreviation used in many dictionaries).
'portia' with letters rearranged gives 'patrio'.
'patrio'+'t'='PATRIOT'
'as' is the link.
(Other definitions for patriot that I've seen before include "One supporting country" , "Flag waver" , "Loyal citizen" , "Loyalist" , "A trip to (anag.)" .)