Cubic capacity (6)
I believe the answer is:
volume
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">cubic capacity</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for volume that I've seen before include "Intensity of sound, in a book?" , "The bulk" , "Book; degree of loudness" , "reading matter?" , "Loud sound or big book" .)