To expose a lie it needs hammering home (4)
I believe the answer is:
nail
This clue is a double definition.
'to expose a lie' is the first definition.
(I've seen this before)
'it needs hammering home' is the second definition.
(I've seen this before)
(Other definitions for nail that I've seen before include "Fixer (on finger?)" , "Detect or catch (a criminal) (colloq.)" , "Growth on digit" , "Hit this on the head to be bang on" , "Bring down" .)