Set off to find film as part of present (8)
I believe the answer is:
detonate
'set off' is the definition.
(synonyms)
'film as part of present' is the wordplay.
'film' becomes 'ET' (E.T. the Extra-Terrestrial).
'as part of' indicates putting letters inside.
'present' becomes 'donate' (donating is a kind of presenting).
'et' put into 'donate' is 'DETONATE'.
'to find' is the link.
(Other definitions for detonate that I've seen before include "Blow up, explode" , "Discharge" , "Start 15 [EXPLOSION]" , "Set off explosive device" , "Trigger" .)