Prefix meaning "false" (6)
I believe the answer is:
pseudo
'prefix meaning false' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for pseudo that I've seen before include "Fake, mock, false" , "Ersatz" , "Use pod (anag.)" , "Pretentious, insincere" , "'Fake, sham (6)'" .)