Jokes about youth with rare finery (4,4)
I believe the answer is:
glad rags
'finery' is the definition.
(both can mean formal clothing)
'jokes about youth with rare' is the wordplay.
'jokes' becomes 'gags' ('gag' can be a synonym of 'joke').
'about' means one lot of letters goes inside another.
'youth' becomes 'lad' (I've seen this before).
'with rare' becomes 'r' (listed by Oxford Dictionary of Abbreviations. I am not sure about the 'with' bit.).
'lad'+'r'='ladr'
'gags' going around 'ladr' is 'GLAD RAGS'.
(Other definitions for glad rags that I've seen before include "Best bib and tucker" , "best suit" , "Fancy clothes for laggards" , "Special kit" , "What partygoers showed up in" .)