Show to be false (6)
I believe the answer is:
debunk
'show to be false' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for debunk that I've seen before include "Expose pretensions of" , "Explode pretensions, myth, of" , "Show to be false or unjustified" , "Expose false claim" , "Expose (pretentious claims?)" .)