European city is area under pressure (4)
I believe the answer is:
pisa
'european city' is the definition.
'is area under pressure' is the wordplay.
'area' becomes 'a' (maths abbreviation).
'under' says to put letters next to each other.
'pressure' becomes 'p' (abbreviation).
'is'+'a'='isa'
'isa' after 'p' is 'PISA'.
(Other definitions for pisa that I've seen before include "City in Tuscany" , "much-visited city" , "Tuscan city" , "City on the River Arno" , "Site of the famous leaning tower" .)