Rubbish I must think should be placed under bed (6)
I believe the answer is:
bunkum
'placed under bed' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'rubbish i must think' is the wordplay.
'rubbish' becomes 'bunk' (I've seen this before).
'i must think' becomes 'um' (expression of hesitation).
'bunk'+'um'='BUNKUM'
'should be' acts as a link.
(Other definitions for bunkum that I've seen before include "cobblers" , "Hogwash, rubbish" , "Ludicrously false statements" , "Drivel" , "Codswallop, hogwash" .)