Rubbish place to sleep? I'm not sure (6)
'rubbish' is the definition.
'place to sleep? i'm not sure' is the wordplay.
'place to sleep?' becomes 'bunk' (I can't justify this - if you can you should give a lot more credence to this answer).
'i'm not sure' becomes 'um' (expression of hesitation).
Can you help me to learn more?
(Other definitions for bunkum that I've seen before include "bombast", "Hot air", "Ludicrously false statements", "cobblers", "Drivel".)