If one interrupts the examiner, there will be a witness (9)
I believe the answer is:
testifier
'a witness' is the definition.
(I know that testifier is a type of witness)
'if one interrupts the examiner' is the wordplay.
'one' becomes 'i' (Roman numeral).
'interrupts' indicates putting letters inside.
'the examiner' becomes 'tester' ('tester' can be a synonym of 'examiner').
'if'+'i'='ifi'
'ifi' placed inside 'tester' is 'TESTIFIER'.
'there will be' acts as a link.
(Other definitions for testifier that I've seen before include "One who may declare" , "Witness" , "person in court?" , "One acknowledging" .)