Duke eclipsing king and cardinal (5)
I believe the answer is:
first
'cardinal' is the definition.
(cardinal can mean the first or most important)
'duke eclipsing king' is the wordplay.
'duke' becomes 'fist' (put up your dukes).
'eclipsing' indicates putting letters inside (eclipse can mean to cover up or hide).
'king' becomes 'R' (abbreviation for rex, king in Latin).
'fist' going around 'r' is 'FIRST'.
'and' acts as a link.
(Other definitions for first that I've seen before include "higher than the others" , "best" , "Preliminary" , "Position for gold" , "Rifts of supreme sort" .)