Exercise when nothing gets off the ground, there being a fog (3,4)
I believe the answer is:
pea soup
'a fog' is the definition.
(pea soup is a kind of fog)
'exercise when nothing gets off the ground' is the wordplay.
'exercise' becomes 'pe' (Physical Education).
'when' becomes 'as'.
'nothing' becomes 'o' (looks like zero - 0).
'gets' says to put letters next to each other.
'off the ground' becomes 'up'.
'pe'+'as'+'o'+'up'='PEA-SOUP'
'there being' is the link.
(Other definitions for pea soup that I've seen before include "Green potage" , "Green broth" , "Liquid vegetable-based dish (like thick fog)" , "Thick vegetable broth" , "dense fog" .)