Crystalline mineral reduced, alongside old book of a certain size (6)
I believe the answer is:
quarto
'book of a certain size' is the definition.
'crystalline mineral reduced alongside old' is the wordplay.
'crystalline mineral' becomes 'quartz' (I've seen this before).
'reduced' means to remove the last letter.
'alongside' is a charade indicator (letters next to each other).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'quartz' with its final letter removed is 'quart'.
'quart'+'o'='QUARTO'
(Other definitions for quarto that I've seen before include "Publishing term, four leaves are formed from one sheet" , "Book page size" , "Ten-by-eight inch paper" , "Twice-folded paper size" , "Book size resulting in folding one sheet four times" .)