Not working with film derivative (8)
I believe the answer is:
offshoot
'derivative' is the definition.
(I've seen this before)
'not working with film' is the wordplay.
'not working' becomes 'off' (I've seen this before).
'with' is a charade indicator (letters next to each other).
'film' becomes 'shoot' ('shoot' can be a synonym of 'film'**).
'off'+'shoot'='OFFSHOOT'
(Other definitions for offshoot that I've seen before include "Subsidiary product or branch" , "Side growth" , "Development" , "Derivative; scion" , "Outgrowth or by-product" .)