Shark vessel circles swirling fog (7)
I believe the answer is:
dogfish
'shark' is the definition.
(dogfish is a kind of shark)
'vessel circles swirling fog' is the wordplay.
'vessel' becomes 'dish' ().
'circles' is an insertion indicator.
'swirling' is an anagram indicator.
'fog' is an anagram of 'ogf'.
'dish' going around 'ogf' is 'DOGFISH'.
(Other definitions for dogfish that I've seen before include "Small, edible shark" , "Restaurants call it rock salmon" , "Nursehound" , "'Small shark, sounds canine (7)'" .)