Horror doesn’t start with online slip (5)
I believe the answer is:
error
'slip' is the definition.
(slip can mean an error or mistake)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for error that I've seen before include "Faux pas" , "Solecism" , "Slip, mistake" , "Slip-up" , "Trial and ___" .)