In revision, keep a tight rein on short story that's mostly barbaric (6)
I believe the answer is:
brutal
'barbaric' is the definition.
(thesaurus)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for brutal that I've seen before include "Vicious" , "Callous" , "Disposed to inflict pain" , "Senselessly cruel" , "Cruel, animal-like" .)