One cannot be sure if such a situation exists (5)
I believe the answer is:
risky
'one' is the definition.
Both the answer and definition are adjectives. Maybe there's an association between them I don't understand?
'cannot be sure if such a situation exists' is the wordplay.
I cannot really see how this works, but
'exists' could be 'is' (if something 'is', it exists) and 'is' is located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for risky that I've seen before include "taking a chance" , "too adventurous" , "Having possibility of danger or loss" , "Potentially dangerous" , "not playing safe" .)