Letter its writer shortened put in English post (7)
I believe the answer is:
epistle
'letter' is the definition.
(I know that letter can be written as epistle)
'shortened put in english post' is the wordplay.
'shortened put' becomes 'st' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'in' means one lot of letters goes inside another.
'english' becomes 'e' (abbreviation).
'post' becomes 'pile' (I've seen this before).
'e'+'pile'='epile'
'st' going inside 'epile' is 'EPISTLE'.
'its writer' acts as a link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for epistle that I've seen before include "verse composition" , "screed" , "Romans perhaps" , "Maybe Paul's" , "Letter, in the Bible perhaps" .)