Expose as a lie Nationalist trouble (4)
I believe the answer is:
nail
'expose as a lie' is the definition.
(I've seen this before)
'nationalist trouble' is the wordplay.
'nationalist' becomes 'n' (abbreviation in politics).
'trouble' becomes 'ail' ('ail' can be a synonym of 'trouble'**).
'n'+'ail'='NAIL'
(Other definitions for nail that I've seen before include "Fixing device" , "Hits this in the head to be bang-on" , "Bring down" , "A metal pin or spike" , "Finger end; fasten" .)