Not casual (6)
I believe the answer is:
dressy
'not casual' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for dressy that I've seen before include "Elaborate" , "Wearing stylish clothing" , "Nattily attired, wearing stylish clothes" , "Smartly clothed" , "Wearing stylish clothes, elegant (6)" .)