Leading duke entertains king (5)
I believe the answer is:
first
'leading' is the definition.
(thesaurus)
'duke entertains king' is the wordplay.
'duke' becomes 'fist' (put up your dukes).
'entertains' is an insertion indicator.
'king' becomes 'r' (r is abbreviation for rex).
'fist' going around 'r' is 'FIRST'.
(Other definitions for first that I've seen before include "to begin with" , "Pre-eminent" , "at the outset" , "higher than the others" , "Top degree class" .)