Glum screening film providing diversion (6)
I believe the answer is:
detour
'diversion' is the definition.
(I've seen this before)
'glum screening film' is the wordplay.
'glum' becomes 'dour' (synonyms).
'screening' means one lot of letters goes inside another (inserted letters are covered or screened by others).
'film' becomes 'ET' (E.T. the Extra-Terrestrial).
'dour' enclosing 'et' is 'DETOUR'.
'providing' is the link.
(Other definitions for detour that I've seen before include "The scenic route?" , "Temporary roundabout way" , "Indirect route" , "Diversion" , "Go round an obstruction" .)