Heavy volume, or medium, in parts of Busoni switched (7)
I believe the answer is:
omnibus
'heavy volume' is the definition.
(large book or volume)
'in parts of busoni switched' is the wordplay.
'in' indicates putting letters inside.
'parts of' indicates anagramming the letters.
'switched' becomes 'm' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'busoni' anagrammed gives 'onibus'.
'onibus' placed around 'm' is 'OMNIBUS'.
'or medium' acts as a link.
This may not be correct. Some or all of it may belong to another bit of the clue.
(Other definitions for omnibus that I've seen before include "Extended programme" , "A number of books in one" , "Multi-part edition" , "Collection of related works reprinted in one volume" , "Old (Clapham?) transport" .)