Frightful rotter makes angry reply (6)
I believe the answer is:
retort
'angry reply' is the definition.
(retort can mean an angry reply or response)
'frightful rotter' is the wordplay.
'frightful' indicates anagramming the letters (a frightful spelling).
'rotter' anagrammed gives 'RETORT'.
'makes' is the link.
(Other definitions for retort that I've seen before include "Sharp riposte" , "swift return" , "Quick and clever or angry reply" , "Riposte, reply" , "Lab equipment" .)