Austere English composer's blocking film (7)
I believe the answer is:
earnest
'austere' is the definition.
(thesaurus)
'english composer's blocking film' is the wordplay.
'english composer's' becomes 'Arne's' (Thomas Arne).
'blocking' indicates putting letters inside.
'film' becomes 'ET' (E.T. the Extra-Terrestrial).
'arnes' going within 'et' is 'EARNEST'.
(Other definitions for earnest that I've seen before include "Serious, solemn" , "money put down" , "Intensely serious; token" , "Fixed" , "art collector" .)