Its true I alter a variety of printed matter (10)
I believe the answer is:
literature
'printed matter' is the definition.
(I've seen this before)
'its true i alter' is the wordplay.
'its' becomes 'liter' (I am not sure about this - if you are sure you should believe this answer much more).
'i' becomes 'a'.
'alter' indicates anagramming the letters.
'true'+'a'='truea'
'truea' with letters rearranged gives 'ature'.
'liter'+'ature'='LITERATURE'
'a variety of' is the 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 literature that I've seen before include "Reading matter" , "Books, writing, etc" , "writer's work" , "fliers" , "Creative writing" .)