Size of book I found after month in Italian cathedral (9)
I believe the answer is:
duodecimo
'size of book' is the definition.
(I've seen this before)
'i found after month in italian cathedral' is the wordplay.
'found after' says to put letters next to each other.
'month' becomes 'dec' (Dec is an example).
'in' means one lot of letters goes inside another.
'italian cathedral' becomes 'duomo' (Italian term for a cathedral).
'i' after 'dec' is 'deci'.
'deci' inserted into 'duomo' is 'DUODECIMO'.
(Other definitions for duodecimo that I've seen before include "Size of book" , "small book" , "scale of work?" .)