Jokes about boy king's finery (4,4)
I believe the answer is:
glad rags
'finery' is the definition.
(both can mean formal clothing)
'jokes about boy king's' is the wordplay.
'jokes' becomes 'gags' (gag can mean a joke).
'about' is an insertion indicator.
'boy' becomes 'lad' (informal term for boy).
'king' becomes 'R' (abbreviation for rex, king in Latin).
'lad'+'r'='ladr'
'gags' enclosing 'ladr' is 'GLAD RAGS'.
(Other definitions for glad rags that I've seen before include "Dressy clothes - happy threads" , "Best clothes worn for special occasions" , "Special kit" , "Fancy clothes for laggards" , "Best bib and tucker" .)