Runs what’s too short, revolting horror film (7)

'horror film' is the definition.
(I know that slasher is a type of horror film)

'runs what's too short revolting' is the wordplay.
'runs' becomes 'r' (cricket abbreviation).
'what's' becomes 'ehs' (sound used to ask 'what?' or 'pardon?').
'too' becomes 'also' (synonyms).
'short' means to remove the last letter.
'revolting' says the letters should be written backwards.
'also' with its final letter taken off is 'als'.
'r'+'ehs'+'als'='rehsals'
'rehsals' written backwards gives 'SLASHER'.