Some retiring parrot, a timid mimic (8)
I believe the answer is:
imitator
'mimic' is the definition.
(I know that mimic can be written as imitator)
'some retiring parrot a timid' is the wordplay.
'retiring' says the letters should be written in reverse.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'rotatimi' is hidden in the letters of 'parrot a timid'.
'rotatimi' written backwards gives 'IMITATOR'.
(Other definitions for imitator that I've seen before include "Copycat" , "parrot" , "One who copies another" , "Ape" , "Copier (of another)" .)