Boy right to crack jokes in finery (4,4)
I believe the answer is:
glad rags
'finery' is the definition.
(both can mean formal clothing)
'boy right to crack jokes' is the wordplay.
'boy' becomes 'lad' (informal term for boy).
'right' becomes 'r' (common abbreviation).
'to crack' means one lot of letters goes inside another (inserted letters crack into others).
'jokes' becomes 'gags' (gag can mean a joke).
'lad'+'r'='ladr'
'ladr' inserted within 'gags' is 'GLAD RAGS'.
'in' acts as a link.
(Other definitions for glad rags that I've seen before include "Smart clothes (for a special occasion)" , "Fancy clothes for laggards" , "Best bib and tucker" , "Dressy clothes - happy threads" , "Dressy clothes in happy shreds" .)