Swirling mist restricts Rugby: an obvious fact (6)
I believe the answer is:
truism
'an obvious fact' is the definition.
(I've seen this in another clue)
'swirling mist restricts rugby' is the wordplay.
'swirling' indicates an anagram.
'restricts' indicates putting letters inside.
'rugby' becomes 'RU'.
'mist' with letters rearranged gives 'tism'.
'tism' placed around 'ru' is 'TRUISM'.
(Other definitions for truism that I've seen before include "Obvious statement that says nothing new or interesting" , "Bromide" , "Self-evident saying" , "Something obvious" , "An obviously accurate statement" .)