Unusually sober attire (5)
I believe the answer is:
robes
'attire' is the definition.
(I've seen this before)
'unusually sober' is the wordplay.
'unusually' indicates anagramming the letters.
'sober' with letters rearranged gives 'ROBES'.
(Other definitions for robes that I've seen before include "In official costume" , "Long loose flowing garments" , "trappings of office?" , "'The 19 across 1 across' of the French Revolution" , "Sober (anag)" .)