Rejecting some parliamentary post (4)
I believe the answer is:
'post' is the definition.
(I know that post can be written as mail)
'rejecting some parliamentary' is the wordplay.
'rejecting' says the letters should be written backwards.
'some' indicates the answer is hidden within the clue (some parts of the word are used).
'liam' is hidden in the letters of 'parliamentary'.
'liam' in reverse letter order is 'MAIL'.
(Other definitions for mail that I've seen before include "Chain armour" , "Armour; posted items" , "Post or body armour" , "Send message" , "Letters and parcels" .)