Guess that's unreliable (7)
I believe the answer is:
suspect
This might be a double definition.
'guess' is the first definition.
(I know that suspect is a type of guess)
'unreliable' is the second definition.
Both the answer and definition are adjectives. Perhaps you can see an association between them that I can't see?
'that's' is the link.
Can you help me to learn more?
(Other definitions for suspect that I've seen before include "Doubt; mistrust" , "Imagine to be true" , "person in the dock" , "One is believed to be guilty but without proof" , "Questionable" .)