Jokes about young man beginning to row in Sunday best (4,4)
I believe the answer is:
glad rags
'sunday best' is the definition.
(both can mean fancy clothes)
'jokes about young man beginning to row' is the wordplay.
'jokes' becomes 'gags' (gag can mean a joke).
'about' indicates putting letters inside.
'young man' becomes 'lad' (lad can mean a boy or young man).
'beginning to' says to take the initial letters.
The first letter of 'row' is 'r'.
'lad'+'r'='ladr'
'gags' going around 'ladr' is 'GLAD RAGS'.
'in' acts as a link.
(Other definitions for glad rags that I've seen before include "What partygoers showed up in" , "Fancy clothes for laggards" , "Smart clothes (for a special occasion)" , "Dressy clothes - happy threads" , "Dressy clothes in happy shreds" .)