Rubbish teacher facing the sack after recession (6)
I believe the answer is:
debris
'rubbish' is the definition.
(I know that debris is a type of rubbish)
'teacher facing the sack after recession' is the wordplay.
'teacher' becomes 'sir' (way of addressing a male teacher).
'facing' says to put letters next to each other.
'the sack' becomes 'bed' (as in hitting the sack).
'after recession' is a reversal indicator.
'sir'+'bed'='sirbed'
'sirbed' back-to-front is 'DEBRIS'.
(Other definitions for debris that I've seen before include "Broken remains" , "Rubble, lots of broken pieces" , "'Ruins, rubble (6)'" , "Rubble, heap of broken pieces" , "Evidence of destruction" .)