A good mimic offering astonished expression (5)
I believe the answer is:
agape
'astonished expression' is the definition.
'agape' can be an answer for 'astonished' (I've seen this before). I'm not sure about the 'expression' bit.
'a good mimic' is the wordplay.
'good' becomes 'g' (abbreviation).
'mimic' becomes 'ape' (ape can mean to copy or imitate).
'a'+'g'+'ape'='AGAPE'
'offering' acts as a link.
Can you help me to learn more?
(Other definitions for agape that I've seen before include "the love of God!" , "showing disbelief" , "ecclesiastical feast" , "love a Christian shows" , "(Of mouth) wide open" .)