The most common and most likely answer to this crossword clue is the 6 letter word CUBAGE
I've seen this crossword clue in The Guardian
C
U
B
A
G
E
Buddy explains!

The straight part
We know that "Capacity" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case capacity = cubage.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.
What does CUBAGE mean?
CUBAGE noun- A cubic measurement.