Retreating inside, upset a timid ape (7)
I believe the answer is:
imitate
'ape' is the definition.
(I know that ape is a more specific form of mock)
'retreating inside upset a timid' is the wordplay.
'retreating' is a reversal indicator.
'inside' indicates a hidden word.
'etatimi' is hidden within 'upset a timid'.
'etatimi' reversed gives 'IMITATE'.
(Other definitions for imitate that I've seen before include "I tame it to copy it" , "Copy or mimic" , "Take as a model" , "Mimic or follow as a model" , "Follow suit" .)