One dubbed plot 'retrograde detritus' (6)
I believe the answer is:
debris
'detritus' is the definition.
(synonyms)
'one dubbed plot retrograde' is the wordplay.
'one dubbed' becomes 'sir' (I can't justify this - if you can you should believe this answer much more).
'plot' becomes 'bed' (a plot of land).
'retrograde' is a reversal indicator.
'sir'+'bed'='sirbed'
'sirbed' back-to-front is 'DEBRIS'.
Can you help me to learn more?
(Other definitions for debris that I've seen before include "Scattered wreckage" , "Rocky fragments" , "Broken remains" , "Refuse remains" , "Evidence of destruction" .)