Alpha's opposite (5)
'alpha's opposite' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for omega that I've seen before include "Last thing" , "Ending of a series or sequence" , "symbol of resistance?" , "Athenian character" , "24th Greek letter" .)
(Other definitions for gowns that I've seen before include "academic gear" , "Scholars' robes" , "formal attire" , "Long flowing dresses for women" , "Fancy frocks" .)