The most common and most likely answer to this crossword clue is the 5 plus 10 letter word CUBIC CENTIMETRE
I've seen this crossword clue in The Sunday Telegraph
C
U
B
I
C
C
E
N
T
I
M
E
T
R
E
Buddy explains!
The straight part
We know that "Small volume" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case small volume = cubic centimetre.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.