Rubbish heap's gathered with great volume (6)
I believe the answer is:
piffle
'rubbish' is the definition.
(I've seen this before)
'heap's gathered with great volume' is the wordplay.
'heap' becomes 'pile' (synonyms).
'gathered' is an insertion indicator.
'with great volume' becomes 'ff' (I am not sure about this - if you are sure you should believe this answer much more).
'pile' going around 'ff' is 'PIFFLE'.
Can you help me to learn more?
(Other definitions for piffle that I've seen before include "Twaddle" , "Cock" , "Trivial nonsense" , "Hogwash" , "Rubbish, nonsense" .)