Tell me, is that believable? (1,3,3)
I believe the answer is:
i ask you
'is that believable?' is the definition.
(expression of disbelief)
I don't understand how the rest of the clue works.
(Other definitions for i ask you that I've seen before include "that's hard to credit" , "interrogator clarifies roles?" , "Can one believe it!" , "how ridiculous is that?" , "really!" .)