Bum has soon, when whipped, come clean (7)
I believe the answer is:
unbosom
'come clean' is the definition.
(to unbosom is to reveal one's secrets)
'bum has soon when whipped' is the wordplay.
'has' says to put letters next to each other.
'when whipped' indicates anagramming the letters (letters whipped into a new form).
'bum' after 'soon' is 'soonbum'.
'soonbum' anagrammed gives 'UNBOSOM'.
(Other definitions for unbosom that I've seen before include "Reveal thoughts - get them off your chest" , "Relieve oneself (of troubling information)" .)