Jokes about juvenile Romeo's Sunday best (4,4)
I believe the answer is:
glad rags
'sunday best' is the definition.
(both can mean fancy clothes)
'jokes about juvenile romeo's' is the wordplay.
'jokes' becomes 'gags' (gag can mean a joke).
'about' is an insertion indicator.
'juvenile' becomes 'lad' (lad can informally mean a young boy).
'romeo' becomes 'R' (phonetic alphabet: alpha, bravo, charlie etc.).
'lad'+'r'='ladr'
'gags' going around 'ladr' is 'GLAD RAGS'.
(Other definitions for glad rags that I've seen before include "best suit" , "Best clothes worn for special occasions" , "What partygoers showed up in" , "Dressy clothes in happy shreds" , "Special kit" .)