Small group generating sheer volume outside (9)
I believe the answer is:
threesome
'small group' is the definition.
(I've seen this before)
'generating sheer volume outside' is the wordplay.
'generating' indicates an anagram (I've seen 'generated' mean this).
'volume' becomes 'tome' (a tome is a large or heavy book).
'outside' is an insertion indicator (some letters go outside others).
'sheer' is an anagram of 'hrees'.
'hrees' inserted inside 'tome' is 'THREESOME'.
(Other definitions for threesome that I've seen before include "Trio" , "a crowd, perhaps" , "triplets, perhaps?" , "Triad" , "Those wise men could be" .)