More substantial volume? More rapid, but not loud (6)
I believe the answer is:
vaster
'more substantial' is the definition.
Both the definition and answer are comparative adjectives. Perhaps there's a link between them I don't understand?
'volume? more rapid but not loud' is the wordplay.
'volume?' becomes 'v' (abbreviation for volume in book titles).
'more rapid' becomes 'faster' ('fast' can be similar in meaning to 'rapid').
'but not' suggests deleting specific letters.
'loud' becomes 'f' (short for forte - 'loud' in music).
'faster' with 'f' removed is 'aster'.
'v'+'aster'='VASTER'
Can you help me to learn more?
(Another definition for vaster that I've seen is " Larger".)