Mentally prepare for endless horror film (5)
I believe the answer is:
psych
'mentally prepare' is the definition.
I can't judge whether this defines the answer.
'endless horror film' is the wordplay.
'endless' means to remove the last letter.
'horror film' becomes 'Psycho' (Alfred Hitchcock film).
'psycho' with its final letter removed is 'PSYCH'.
'for' acts as a link.
Can you help me to learn more?
(Other definitions for psych that I've seen before include "Stimulate" , "Mind doctor (colloquial abbreviation)" , "Intimidate someone by making him feel uneasy" , "To subject to mind games" .)