Of course it will happen (4,5)
I believe the answer is:
sure thing
'of course' is the definition.
(I've seen this in another clue)
'it will happen' is the wordplay.
I cannot quite see how this works, but
'it' could be 'thing' and 'thing' is found in the answer.
The remaining letters 'sure' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for sure thing that I've seen before include "no chance of losing" , "of course" , "A certainty - I agree!" , "cert" .)