So, pretence collapses – everyone knows it! (4,6)
I believe the answer is:
open secret
'everyone knows it' is the definition.
(something everybody secretly knows)
'so pretence collapses' is the wordplay.
'collapses' indicates an anagram.
'so'+'pretence'='sopretence'
'sopretence' with letters rearranged gives 'OPEN SECRET'.
(Other definitions for open secret that I've seen before include "common knowledge" , "Generally known matter" , "Private matter actually known to many" , "Everyone knows it" , "Centre pose (anag.)" .)