Putting a number in an envelope saying "Authorship Unknown" (4)
I believe the answer is:
anon
'unknown' is the definition.
(I've seen this before)
'putting a number in an' is the wordplay.
'putting a number' becomes 'no' (I can't explain this - if you can you should believe this answer much more).
'in' means one lot of letters goes inside another.
'no' going inside 'an' is 'ANON'.
'envelope saying authorship' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for anon that I've seen before include "Presently - or unknown" , "Immediately" , "In the near future" , "soon to be written" , "Soon; unattributed" .)