Fabricator set up train line (4)
I believe the answer is:
liar
'fabricator' is the definition.
(I've seen this in another clue)
'set up train line' is the wordplay.
'set up' shows that the letters should be reversed in order (in a down clue, letters go up).
'train line' becomes 'rail' (I've seen this in another clue).
'rail' backwards is 'LIAR'.
(Other definitions for liar that I've seen before include "Dissembler" , "Fibber" , "Dishonest person" , "Person telling stories" , "27 [DECEIVER]" .)