Surrounded by 500 in a film (6)
I believe the answer is:
amidst
'surrounded by' is the definition.
(amidst means within or surrounded by)
'500 in a film' is the wordplay.
'500' becomes 'D' (Roman numeral for 500).
'in' indicates putting letters inside.
'film' becomes 'mist' (to film over is to mist or fog over).
'a'+'mist'='amist'
'd' inserted within 'amist' is 'AMIDST'.
(Other definitions for amidst that I've seen before include "Beset" , "contained" , "Embraced by" , "Among" , "Surrounded by" .)