Two teachers picked up rubbish (6)
I believe the answer is:
debris
'rubbish' is the definition.
(debris is a kind of rubbish)
'two teachers picked up' is the wordplay.
'two teachers' means two substitutions for 'teacher'.
'teacher' becomes 'sir' (way of addressing a male teacher).
'teacher' becomes 'BEd' (holder of a BEd, Bachelor of Education degree).
'picked up' says the letters should be written in reverse.
'sir'+'bed'='sirbed'
'sirbed' in reverse letter order is 'DEBRIS'.
(Other definitions for debris that I've seen before include "Scattered rubbish" , "Fragments of wreckage" , "Scattered wreckage" , "Broken remains" , "Rubble, lots of broken pieces" .)