Most obscure desk art rubbished (7)
I believe the answer is:
darkest
'most obscure' is the definition.
('dark' can be similar in meaning to 'obscure')
'desk art rubbished' is the wordplay.
'rubbished' indicates an anagram.
'desk'+'art'='deskart'
'deskart' with letters rearranged gives 'DARKEST'.
(Other definitions for darkest that I've seen before include "Gloomiest or blackest" , "Most dim" , "Most gloomy" , "Most devoid of light" , "'The . . . . . . . hour comes before the dawn'" .)