Posterior and chest seen, sadly in secret (6,3,6)
I believe the answer is:
behind the scenes
'in secret' is the definition.
'posterior and chest seen sadly' is the wordplay.
'posterior' becomes 'behind' ('behind' can be a synonym of 'posterior'**).
'and' says to put letters next to each other.
'sadly' indicates anagramming the letters.
'chest'+'seen'='chestseen'
'chestseen' anagrammed gives 'thescenes'.
'behind'+'thescenes'='BEHIND THE SCENES'
(Other definitions for behind the scenes that I've seen before include "responsible for acts, partly" , "Privately" , "in the private sector" , "backstage" , "Playwright is" .)