Enclosure's scrap of paper reveals author's mistake (4,2,3,3)
I believe the answer is:
slip of the pen
'enclosure's scrap' is the definition.
I don't know anything about this answer so I cannot tell whether it can be defined by this definition.
'paper reveals author's mistake' is the wordplay.
I cannot quite see how this works, but
'paper' could be 'slipof' (slip of paper is a kind of paper) and 'slipof' is present in the answer.
'author' could be 'pen' and 'pen' is found within the answer.
The remaining letters 'the' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'of' is the link.
Can you help me to learn more?
(Other definitions for slip of the pen that I've seen before include "Writing mistake" , "Error in writing" , "Something written in error" , "Clerical error" , "Accidental written mistake" .)