I am changing Rita to become a mimic (8)
I believe the answer is:
imitator
'a mimic' is the definition.
(I know that mimic can be written as imitator)
'i am changing rita to' is the wordplay.
'i am' becomes 'I'm' (contraction).
'changing' indicates an anagram.
'rita'+'to'='ritato'
'ritato' anagrammed gives 'itator'.
'im'+'itator'='IMITATOR'
'become' acts as a link.
(Other definitions for imitator that I've seen before include "A trio, Tim, is a mimic" , "follower" , "Ape" , "Copycat" , "One who mimics or emulates" .)