He should always be prepared to offer a challenge (8)
I believe the answer is:
sentinel
'he should always be prepared to offer a challenge' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
This is all the clue.
Can you help me to learn more?
(Other definitions for sentinel that I've seen before include "Watchperson" , "Guard whose job is to stand and keep watch" , "One watching" , "Soldier keeping watch" , "Look-out, military guard" .)