English lord established as most senior (6)
I believe the answer is:
eldest
'most senior' is the definition.
(I've seen this before)
'english lord established' is the wordplay.
'english' becomes 'e' (abbreviation).
'lord' becomes 'ld' (abbreviation for the aristocratic title).
'established' becomes 'est' (I've seen this in other clues).
'e'+'ld'+'est'='ELDEST'
'as' acts as a link.
(Other definitions for eldest that I've seen before include "Most old" , "Most senior of a group" , "First in the order of birth" , "Most advanced in years" , "The oldest of three or more" .)