Everyone that is joining duke is in league (6)
I believe the answer is:
allied
'in league' is the definition.
(I've seen this before)
'everyone that is joining duke' is the wordplay.
'everyone' becomes 'all' (all people).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'joining' is a charade indicator (letters next to each other).
'duke' becomes 'd' (abbreviation).
'all'+'ie'+'d'='ALLIED'
'is' acts as a link.
(Other definitions for allied that I've seen before include "Joined in friendship, treaty, etc" , "Joined for mutual benefit" , "(Of nations) working together" , "United through treaty" , "Joined by treaty or agreement" .)