Likely to happen but not certain (8)
I believe the answer is:
probable
'likely to happen but not certain' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for probable that I've seen before include "Seeming to have a good chance of happening" , "Likely to be the case" , "Like to happen" , "Person most likely" , "on the cards" .)