In the middle of a film starting to dig in (6)
I believe the answer is:
amidst
'in the middle of' is the definition.
(I've seen this before)
'a film starting to dig in' is the wordplay.
'film' becomes 'mist' (I've seen this before).
'starting to' suggests taking the first letters.
'in' means one lot of letters goes inside another.
The initial letter of 'dig' is 'd'.
'mist' going around 'd' is 'midst'.
'a'+'midst'='AMIDST'
(Other definitions for amidst that I've seen before include "In the thick of" , "Admits (anag.)" , "During; within" , "In the middle of the morning I would have a piece of crust" , "Beset" .)