It won't stop you reading it! (6)
I believe the answer is:
letter
'it' is the definition.
The definition suggests a singular noun which matches the answer.
'it won't stop you reading' is the wordplay.
I cannot quite understand how this works, but
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is present in the answer.
'reading' could be 'r' (one of the traditional three R's of education) and 'r' is present in the answer.
'won't stop you' could be 'let' (I've seen this before) and 'let' is found within the answer.
A single letter 'e' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for letter that I've seen before include "reference?" , "Eg, A" , "Form of correspondence" , "Character - landlord - epistle" , "Piece of correspondence" .)