William — first king to have Henry’s crown (6)
I believe the answer is:
wreath
'crown' is the definition.
(thesaurus)
'william first king to have henry's' is the wordplay.
'william first' becomes 'w' (1st letter of 'william').
'king' becomes 'r' (r is abbreviation for rex).
'to have' becomes 'eat' (eating is a kind of having).
'henry' becomes 'h' (symbol for the unit of electrical inductance).
'w'+'r'+'eat'+'h'='WREATH'
(Other definitions for wreath that I've seen before include "Flower arrangement" , "Circle of flowers for mourning" , "26 Down may be shaped into this decorative ring" , "Circlet of flowers" , "Circular band of flowers and leaves, often seen on graves" .)