Pretence so false, common knowledge (4,6)
I believe the answer is:
open secret
'common knowledge' is the definition.
(I've seen this in another clue)
'pretence so false' is the wordplay.
'false' indicates anagramming the letters.
'pretence'+'so'='pretenceso'
'pretenceso' anagrammed gives 'OPEN SECRET'.
(Other definitions for open secret that I've seen before include "Generally known matter" , "Everyone knows it" , "Private matter actually known to many" , "common knowledge" , "Centre pose (anag.)" .)