Repetition terribly irate on it (9)
I believe the answer is:
iteration
'repetition' is the definition.
(I know that iteration is a type of repetition)
'terribly irate on it' is the wordplay.
'terribly' indicates an anagram.
'irate'+'on'+'it'='irateonit'
'irateonit' with letters rearranged gives 'ITERATION'.
(Other definitions for iteration that I've seen before include "Repetition" , "Reprise" , "Repeating" , "Repeats" , "Repeated performing" .)