What is admitted by courageous witness (6)
I believe the answer is:
behold
'witness' is the definition.
(thesaurus)
'what is admitted by courageous' is the wordplay.
'what' becomes 'eh' (I've seen this in other clues).
'is admitted by' is an insertion indicator.
'courageous' becomes 'bold' ('bold' can be similar in meaning to 'courageous').
'eh' inserted inside 'bold' is 'BEHOLD'.
(Other definitions for behold that I've seen before include "Lo - look!" , "Archaic word for see or observe" , "Look at with attention" , "Archaic injunction to observe" , "Lo, see here" .)