Object to sentry's demand for ID (9)
I believe the answer is:
challenge
I believe this clue is a double definition.
'object to' is the first definition.
(thesaurus)
'sentry's demand for id' is the second definition.
(act of a sentry demanding a countersign)
(Other definitions for challenge that I've seen before include "Take issue with" , "difficulty" , "problem" , "Ask for identification" , "call for proof" .)