Ultimate in film celluloid? (7)
I believe the answer is:
plastic
'celluloid?' is the definition.
(celluloid is a kind of plastic)
'ultimate in film' is the wordplay.
'ultimate' becomes 'last' (similar in meaning).
'in' is an insertion indicator.
'film' becomes 'pic' (short for picture).
'last' put into 'pic' is 'PLASTIC'.
(Other definitions for plastic that I've seen before include "Artificial/synthetic" , "non-metallic material" , "Artificial; easily moulded" , "Ubiquitous synthetic material" , "Capable of being moulded" .)