Size of a book or coat changed by Victor and Oscar (6)
I believe the answer is:
octavo
'size of a book' is the definition.
'octavo' can be an answer for 'book' (I've seen this before). I'm not sure about the 'size of a' bit.
'coat changed by victor and oscar' is the wordplay.
'changed' is an anagram indicator.
'by' means one lot of letters go next to another.
'victor' becomes 'V' (phonetic alphabet: alpha, bravo, charlie etc.).
'and' says to put letters next to each other.
'oscar' becomes 'O' (phonetic alphabet: alpha, bravo, charlie etc.).
'coat' is an anagram of 'octa'.
'octa'+'v'+'o'='OCTAVO'
'or' is the link.
Can you help me to learn more?
(Other definitions for octavo that I've seen before include "An eight-leaved book or page" , "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)" .)