Put nice letters in fast post on the gravy train (8)
I believe the answer is:
sinecure
'post on the gravy train' is the definition.
'sinecure' can be an answer for 'post' (sinecure is a kind of post). I am unsure of the 'on the gravy train' bit.
'put nice letters in' is the wordplay.
'put' becomes 'sure' (I can't justify this - if you can you should give a lot more credence to this answer).
'letters' indicates anagramming the letters.
'in' is an insertion indicator.
'nice' is an anagram of 'inec'.
'sure' placed around 'inec' is 'SINECURE'.
'fast' is the link.
Can you help me to learn more?
(Other definitions for sinecure that I've seen before include "Insecure in cushy job" , "Post requiring little work" , "Soft job, may be insecure" , "Very lucrative post" , "Office that involves minimal duties" .)