Good mimic produces yawn (4)
I believe the answer is:
gape
'yawn' is the definition.
(I know that yawn can be written as gape)
'good mimic' is the wordplay.
'good' becomes 'g' (abbreviation).
'mimic' becomes 'ape' (ape can mean to copy or imitate).
'g'+'ape'='GAPE'
'produces' acts as a link.
(Other definitions for gape that I've seen before include "Stare with open mouth in wonder" , "Yawn" , "Hang wide open" , "Gawp" , "Open-mouthed stare" .)