It's axiomatic that a run can't be finished in swirling mist (6)
I believe the answer is:
truism
'it's axiomatic that a run can't' is the definition.
I can't tell whether this definition defines the answer.
'finished in swirling mist' is the wordplay.
'finished' becomes 'ru' (I can't explain this - if you can you should believe this answer much more).
'in' indicates putting letters inside.
'swirling' indicates an anagram.
'mist' anagrammed gives 'tism'.
'ru' put inside 'tism' is 'TRUISM'.
'be' is the link.
Can you help me to learn more?
(Other definitions for truism that I've seen before include "An obvious truth" , "Self-evident saying" , "Something obvious" , "Bromide" , "Saying that's obviously accurate" .)