It is likely to succeed — rush, get in lively! (4,5)
I believe the answer is:
sure thing
'it is likely to succeed' is the definition.
(something certain to do well)
'rush get in lively' is the wordplay.
'rush get in' anagrams to 'SURE THING'.
Though, I am not clear how this is indicated.
Can you help me to learn more?
(Other definitions for sure thing that I've seen before include "of course" , "A certainty - I agree!" , "no chance of losing" , "cert" .)