Rubbish teacher has qualification returned (6)
I believe the answer is:
debris
'rubbish' is the definition.
(debris is a kind of rubbish)
'teacher has qualification returned' is the wordplay.
'teacher' becomes 'sir' (I've seen this before).
'has qualification' becomes 'bed' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'returned' shows that the letters should be reversed in order.
'sir'+'bed'='sirbed'
'sirbed' reversed gives 'DEBRIS'.
Can you help me to learn more?
(Other definitions for debris that I've seen before include "'Ruins, rubble (6)'" , "Scattered remains" , "Rubble, lots of broken pieces" , "Brides among rubble" , "Rubble, heap of broken pieces" .)