Bearing witness to adjutant being eviscerated is challenging (9)
I believe the answer is:
attesting
'bearing witness' is the definition.
('attest' can be a synonym of 'bear witness')
'adjutant being eviscerated is challenging' is the wordplay.
'being eviscerated' means to remove the middle letters (the word is disembowelled of its inside letters).
'is challenging' becomes 'testing' (I am not sure about this - if you are sure you should believe this answer much more).
'adjutant' with its centre taken out is 'at'.
'at'+'testing'='ATTESTING'
'to' is the link.
Can you help me to learn more?
(Other definitions for attesting that I've seen before include "Witnessing" , "being a witness" , "Declaring that something is the case" .)