Frigg ultimately appears in this swirling vision (5)
I believe the answer is:
sight
'vision' is the definition.
('sight' can be a synonym of 'vision')
'frigg ultimately appears in this swirling' is the wordplay.
'ultimately' says to take the final letters.
'appears in' is an insertion indicator.
'swirling' indicates an anagram.
The last letter of 'frigg' is 'g'.
'this' anagrammed gives 'siht'.
'g' placed inside 'siht' is 'SIGHT'.
(Other definitions for sight that I've seen before include "Bodily faculty" , "24" , "Field of vision" , "A great many" , "Notice" .)