Good mimic gets one to stare open-mouthed (4)
I believe the answer is:
gape
'one to stare open-mouthed' is the definition.
(I've seen this before)
'good mimic' is the wordplay.
'good' becomes 'g' (abbreviation).
'mimic' becomes 'ape' (I've seen this before).
'g'+'ape'='GAPE'
'gets' is the link.
(Other definitions for gape that I've seen before include "Yawn" , "watch in wonder" , "Appear amazed" , "Stare with open mouth in wonder" , "Hang wide open" .)