Capital's decline in endless spiral (8)
I believe the answer is:
helsinki
'capital's' is the definition.
(capital of Finland)
'decline in endless spiral' is the wordplay.
'decline' becomes 'sink' (sinking is a kind of declining).
'in' means one lot of letters goes inside another.
'endless' means to remove the last letter.
'spiral' becomes 'helix' (synonyms).
'helix' with its last letter removed is 'heli'.
'sink' placed inside 'heli' is 'HELSINKI'.
(Other definitions for helsinki that I've seen before include "Finnish capital" , "1952 Olympic Games venue" , "European capital, venue for the 1952 Olympics" , "Nordic capital" , "Is he link to a European city?" .)