Fast runner briefly interrupted by a fog (4)
I believe the answer is:
haar
'fog' is the definition.
(I know that Haar is a type of fog)
'fast runner briefly interrupted by a' is the wordplay.
'fast runner' becomes 'hare' (I've seen this in another clue).
'briefly' means to remove the last letter.
'interrupted by' is an insertion indicator.
'hare' with its last letter taken away is 'har'.
'har' going around 'a' is 'HAAR'.
(Other definitions for haar that I've seen before include "Raw sea-mist" , "Sea-fog" , "Sea mist" , "chilly sea-mist" , "East-coast sea fog" .)