Surrounded by 500 in a fog (6)
I believe the answer is:
amidst
'surrounded by' is the definition.
(amidst means within or surrounded by)
'500 in a fog' is the wordplay.
'500' becomes 'D' (Roman numeral for 500).
'in' is an insertion indicator.
'fog' becomes 'mist' (mist is a kind of fog).
'a'+'mist'='amist'
'd' placed within 'amist' is 'AMIDST'.
(Other definitions for amidst that I've seen before include "In the centre of" , "In the middle of the morning I would have a piece of crust" , "In the thick of" , "Admits (anag.)" , "Among" .)