Surrounded by a fog enveloping first bit of descent (6)
I believe the answer is:
amidst
'surrounded by' is the definition.
(amidst means within or surrounded by)
'a fog enveloping first bit of descent' is the wordplay.
'fog' becomes 'mist' (mist is a kind of fog).
'enveloping' means one lot of letters goes inside another.
'first bit of descent' becomes 'd' (1st letter of 'descent').
'mist' enclosing 'd' is 'midst'.
'a'+'midst'='AMIDST'
(Other definitions for amidst that I've seen before include "Admits (anag.)" , "Central" , "During; within" , "In the centre of" , "It's mad (anag.)" .)