Admits clown's upsetting them secretly (4,2)
I believe the answer is:
owns up
'admits' is the definition.
(to own up to something is to admit it)
'clown's upsetting them secretly' is the wordplay.
'secretly' indicates the answer is hidden within the clue.
'OWNS UP' is hidden in the letters of 'clowns upsetting them'.
(Other definitions for owns up that I've seen before include "Allows" , "Admits blame" , "'Comes clean, confesses (4,2)'" , "Admits (to something)" , "'Admits, confesses (4,2)'" .)