Fighting witnessed in European capital (6)
I believe the answer is:
warsaw
'european capital' is the definition.
(I've seen this before)
'fighting witnessed' is the wordplay.
'fighting' becomes 'war'.
'witnessed' becomes 'saw' ('see' can be a synonym of 'witness').
'war'+'saw'='WARSAW'
'in' is the link.
(Other definitions for warsaw that I've seen before include "European city (with a Convention for commercial flying)" , "Capital of a European Union state" , "Poland's largest city" , "Capital city on the Vistula" , "Largest city in Poland" .)