Kiosk failing to open? That's hard to believe (4)
I believe the answer is:
tall
'hard to believe' is the definition.
(thesaurus)
'kiosk failing to open?' is the wordplay.
'kiosk' becomes 'stall' (synonyms).
'failing to open?' means to remove the first letter (the opening letter is removed).
'stall' with its first letter removed is 'TALL'.
'that's' is the link.
(Other definitions for tall that I've seen before include "Vertically large" , "Unbelievable" , "Like giants" , "Towering" , "Remarkable" .)