I am to get the post not without some burden (10)
I believe the answer is:
imposition
'some burden' is the definition.
(imposition is a kind of burden)
'i am to get the post' is the wordplay.
'i am' becomes 'I'm' (contraction).
'to get' means one lot of letters go next to another.
'the post' becomes 'position' ('position' can be a synonym of 'post').
'im'+'position'='IMPOSITION'
'not without' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for imposition that I've seen before include "Unwelcome demand or burden" , "tax" .)