Reason for an R rating (4)
I believe the answer is:
gore
'reason for an r rating' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for gore that I've seen before include "Result of bleeding" , "Impale" , "Pierce or wound with a horn or tusk" , "Bloodshed" , "piece of material" .)