Conclusion of hidden depth (3)
I believe the answer is:
end
'conclusion' is the definition.
(something's conclusion is its end)
'of hidden depth' is the wordplay.
'of' indicates a hidden word.
'END' is hidden within 'hidden depth'.
(Other definitions for end that I've seen before include "Discontinue" , "Butt" , "Game at bowls" , "Limit" , "SECOND PART OF 10" .)