Ape astride back of au pair (3)
I believe the answer is:
duo
'pair' is the definition.
(two people)
'ape astride back of au' is the wordplay.
'ape' becomes 'do' (both can mean to mimic).
'astride' indicates putting letters inside.
'back of' suggests the final letters.
The last letter of 'au' is 'u'.
'do' placed around 'u' is 'DUO'.
(Other definitions for duo that I've seen before include "rejected Arab's lute" , "Performing couple" , "Two of a kind" , "Musical work for two" , "Musical pair" .)