Runs what’s too short, revolting horror film (7)
I believe the answer is:
slasher
'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'.
(Other definitions for slasher that I've seen before include "Type of horror movie" , "one criticises severely" .)