Area is ten by ten in film (6)
I believe the answer is:
extent
'area' is the definition.
(I've seen this before)
'ten by ten in film' is the wordplay.
'ten' becomes 'X' (Roman numeral).
'by' means one lot of letters go next to another.
'in' indicates putting letters inside.
'film' becomes 'ET' (E.T. the Extra-Terrestrial).
'x'+'ten'='xten'
'xten' placed inside 'et' is 'EXTENT'.
'is' acts as a link.
(Other definitions for extent that I've seen before include "Bulk - compass" , "stretch" , "Measure" , "Scope, area" , "Degree - amount - scope" .)