Most senior leaders of European lands declare emergency, summon Tsipras (6)
I believe the answer is:
eldest
'most senior' is the definition.
(I've seen this before)
'leaders of european lands declare emergency summon tsipras' is the wordplay.
'leaders of' says to take the initial letters.
The initial letters of 'european lands declare emergency summon tsipras' is 'ELDEST'.
(Other definitions for eldest that I've seen before include "Most senior of a group" , "Most senior in age" , "Most aged" , "First to be born" , "Most senior (E, not O)" .)