I trusted new arrangement but it's rubbish (8)
I believe the answer is:
detritus
'it's rubbish' is the definition.
(detritus is a kind of rubbish)
'i trusted new arrangement' is the wordplay.
'new arrangement' indicates anagramming the letters (I've seen this in other clues).
'i'+'trusted'='itrusted'
'itrusted' anagrammed gives 'DETRITUS'.
'but' acts as a link.
(Other definitions for detritus that I've seen before include "Bits and pieces of rubbish" , "Gravel or loose matter produced by erosion" , "It rusted to become debris" , "Remains" , "Any kind of debris" .)