Like formal attire (6)
I believe the answer is:
'like formal attire' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for dressy that I've seen before include "'Stylish, smart (6)'" , "(Of clothes) smart, formal" , "Stylishly attired" , "Elaborate" , "Wearing stylish clothes, elegant (6)" .)