New sides unusually showing up (6)
I believe the answer is:
unused
'new' is the definition.
(similar in meaning)
'sides unusually showing up' is the wordplay.
'showing' indicates a hidden word (the answer is shown in the clue).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'desunu' is hidden in the letters of 'sides unusually'.
'desunu' in reverse letter order is 'UNUSED'.
(Other definitions for unused that I've seen before include "Mint, virgin" , "hadn't been exploited" , "Neither employed nor second-hand" , "Not accustomed to" , "Extra" .)