The Spanish duke's established to be the most senior (6)
I believe the answer is:
eldest
'the most senior' is the definition.
(I've seen this in another clue)
'the spanish duke's established' is the wordplay.
'the spanish' becomes 'el' ('the' in Spanish).
'duke' becomes 'd' (abbreviation).
'established' becomes 'est' (abbreviation).
'el'+'d'+'est'='ELDEST'
'to be' is the link.
(Other definitions for eldest that I've seen before include "Most advanced in years" , "Most advanced in age, led set (anag.)" , "Most old" , "Most senior of a group" , "Firstborn; led set (anag.)" .)