It's a safe bet their guns will be out of order (4,5)
I believe the answer is:
sure thing
'it's a safe bet' is the definition.
(I've seen this in another clue)
'their guns will be out of order' is the wordplay.
'their guns' can be anagrammed to 'SURE THING'.
But, I'm not sure how the anagram is indicated.
Can you help me to learn more?
(Other definitions for sure thing that I've seen before include "A certainty - I agree!" , "of course" , "no chance of losing" , "cert" .)