Private balls held in court (6)
I believe the answer is:
covert
'private' is the definition.
(thesaurus)
'balls held in court' is the wordplay.
'balls' becomes 'over' (I've seen this in other clues).
'held in' indicates putting letters inside.
'court' becomes 'ct'.
'over' placed into 'ct' is 'COVERT'.
(Other definitions for covert that I've seen before include "Not open; thicket" , "Not acknowledged" , "Secret like fox's lair" , "A fox's retreat, secret" , "Area of undergrowth in which animals hide" .)