Short letter to reveal upset over film (7)
I believe the answer is:
notelet
'short letter' is the definition.
(I have seen 'Short letter' mean 'notelet' so perhaps 'letter' could also mean 'notelet'; 'short' indicates an abbreviation)
'reveal upset over film' is the wordplay.
'reveal' becomes 'leton' (synonyms).
'upset' says the letters should be written in reverse.
'over' means one lot of letters go next to another.
'film' becomes 'ET' (E.T. the Extra-Terrestrial).
'leton' backwards is 'notel'.
'notel'+'et'='NOTELET'
'to' is the link.
(Other definitions for notelet that I've seen before include "Folded sheet for an informal letter" , "Paper for informal letter" , "means of communication" , "Small sheet for letter" , "short written message" .)