Copy a page inside Stern (3)
I believe the answer is:
ape
'copy' is the definition.
(ape can mean to copy or imitate)
'a page inside stern' is the wordplay.
'page' becomes 'p'.
'inside' says to take the centre.
The middle letter of 'stern' is 'e'.
'a'+'p'+'e'='APE'
(Other definitions for ape that I've seen before include "Send up" , "Take off" , "gorilla" , "Parrot" , "Closely follow" .)