It's common knowledge, Frank has a skeleton in the closet (4,6)
I believe the answer is:
open secret
'it's common knowledge' is the definition.
(I've seen this before)
'frank has a skeleton in the closet' is the wordplay.
'frank' becomes 'open' ('open' can be a synonym of 'frank').
'has' is a charade indicator (letters next to each other).
'a skeleton in the closet' becomes 'secret' (I am not sure about this - if you are sure you should believe this answer much more).
'open'+'secret'='OPEN SECRET'
Can you help me to learn more?
(Other definitions for open secret that I've seen before include "common knowledge" , "Centre pose (anag.)" , "Generally known matter" , "Private matter actually known to many" , "Everyone knows it" .)