Decline in large European city (6)
I believe the answer is:
lessen
'decline' is the definition.
(I know that decline is a more specific form of the action lessen)
'large european city' is the wordplay.
'large' becomes 'L' (eg in clothes sizes).
'european city' becomes 'Essen' (city in Germany).
'l'+'essen'='LESSEN'
'in' acts as a link.
(Other definitions for lessen that I've seen before include "Make fewer in number" , "Lower" , "Make smaller or fewer" , "Make smaller for lenses" , "Cut back or diminish" .)