City to decline in endless spiral (8)
I believe the answer is:
helsinki
'city' is the definition.
(Helsinki is an example)
'decline in endless spiral' is the wordplay.
'decline' becomes 'sink' (sinking is a kind of declining**).
'in' is an insertion indicator.
'endless' means to remove the last letter.
'spiral' becomes 'helix' ('helix' can be a synonym of 'spiral').
'helix' with its final letter taken away is 'heli'.
'sink' inserted inside 'heli' is 'HELSINKI'.
'to' is the link.
(Other definitions for helsinki that I've seen before include "European capital, venue for the 1952 Olympics" , "Sikh line (anag)" , "Finland's capital" , "1952 Olympic Games venue" , "foreign capital" .)