Rat lifted rubbish beneath train mostly (7)
I believe the answer is:
traitor
'rat' is the definition.
(I've seen this in another clue)
'lifted rubbish beneath train mostly' is the wordplay.
'lifted' says the letters should be written backwards (going up in down clue).
'rubbish' becomes 'rot' (both can mean nonsense).
'beneath' says to put letters next to each other (in a down clue).
'mostly' means to remove the last letter.
'rot' written backwards gives 'tor'.
'train' with its last letter taken off is 'trai'.
'tor' after 'trai' is 'TRAITOR'.
(Other definitions for traitor that I've seen before include "Person betraying a cause" , "Someone guilty of treason" , "Rat displayed" , "Betrayer, quisling" , "Renegade" .)