Partly concoct a volume having a certain page size (6)
I believe the answer is:
octavo
'page size' is the definition.
(I've seen this before)
'partly concoct a volume having' is the wordplay.
'partly' indicates a hidden word (part of the clue is used).
'OCTAVO' is hidden in the letters of 'concoct a volume having'.
'a certain' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for octavo that I've seen before include "Size of book page" , "Common book size" , "Size of a book, pages are made by folding a sheet of paper three times to form eight leaves (6)" , "An eight-leaved book or page" .)