Unbelieving (7)
'unbelieving' is the definition.
(similar in meaning)
This is all the clue.
(Other definitions for incredulous that I've seen before include "Unwilling to believe something" , "Unable to believe something" , "Sceptical, unbelieving" .)
(Other definitions for cynical that I've seen before include "Of a pessimistic disposition about the world" , "Contemptuous, mocking" , "Disinclined to believe the sincerity of others" , "Believing the worst of motives" , "streetwise" .)