Accumulated debris (8)
I believe the answer is:
detritus
'accumulated debris' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for detritus that I've seen before include "Accumulation of debris - it rusted (anag)" , "Rubbish - it rusted (anag)" , "Gravel or loose matter produced by erosion" , "Loose eroded matter" , "Worn down rocks, gravel etc." .)