Open-mouthed, seeing silver copy (5)
I believe the answer is:
agape
'open-mouthed' is the definition.
(I've seen this in another clue)
'silver copy' is the wordplay.
'silver' becomes 'Ag' (Ag is the chemical symbol for silver).
'copy' becomes 'ape' (ape can mean to copy or imitate).
'ag'+'ape'='AGAPE'
'seeing' acts as a link.
(Other definitions for agape that I've seen before include "showing disbelief" , "love feast" , "(Of mouth) wide open" , "Christian feast" , "the love of God!" .)