Bound to be a sudden increase! (4)
I believe the answer is:
leap
I believe this is a double definition.
'bound' is the first definition.
(I know that bound can be written as leap)
'a sudden increase' is the second definition.
(I've seen this in another clue)
'to be' acts as a link.
(Other definitions for leap that I've seen before include "identify extra day?" , "Jump across" , "'Look before you . . . .'" , "Basket" , "Sort of year" .)