Horror film must occasionally intimidate (5,3)
I believe the answer is:
psych out
'intimidate' is the definition.
(I've seen this in another clue)
'horror film must occasionally' is the wordplay.
'horror film' becomes 'Psycho' (Alfred Hitchcock film).
'occasionally' indicates alternate letters (letters taken at regular intervals or occasions).
The alternate letters of 'must' are 'ut'.
'psycho'+'ut'='PSYCH OUT'
(Other definitions for psych out that I've seen before include "cow" , "intimidate" , "Play mental games" , "Guess correctly the intentions of another (informal)" , "Informally, guess correctly the intentions of another" .)