It is placed in a challenging position (6,3)
I believe the answer is:
sentry box
'it' is the definition.
The definition suggests a singular noun which matches the answer.
'placed in a challenging position' is the wordplay.
I cannot quite understand how this works, but
'placed' could be 'sent' ('send' can be a synonym of 'place') and 'sent' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
'is' is the link.
Can you help me to learn more?
(Other definitions for sentry box that I've seen before include "Guard's shelter" , "Small shelter to protect guard" .)