Constant thick fog ruined contest (9)
I believe the answer is:
cockfight
'contest' is the definition.
(cockfight is a kind of contest)
'constant thick fog ruined' is the wordplay.
'constant' becomes 'c' (mathematical abbreviation).
'ruined' indicates anagramming the letters.
'thick'+'fog'='thickfog'
'thickfog' anagrammed gives 'ockfight'.
'c'+'ockfight'='COCKFIGHT'
(Other definitions for cockfight that I've seen before include "Illegal contest of birds" , "Battle as between roosters" , "Illegal battle" , "in which feathers will fly" , "Violent contest between birds" .)