Finery, Sunday best (4,4)
I believe the answer is:
This clue is a double definition.
'finery' is the first definition.
(both can mean formal clothing)
'sunday best' is the second definition.
(both can mean fancy clothes)
(Other definitions for glad rags that I've seen before include "What partygoers showed up in" , "Dressy clothes in happy shreds" , "Special kit" , "Smart clothes (for a special occasion)" , "Best bib and tucker" .)