Good mimic makes you yawn (4)
I believe the answer is:
gape
'yawn' is the definition.
(synonyms)
'good mimic' is the wordplay.
'good' becomes 'g' (abbreviation).
'mimic' becomes 'ape' (I've seen this before).
'g'+'ape'='GAPE'
'makes you' acts as a link.
(Other definitions for gape that I've seen before include "Failure to meet" , "Appear amazed" , "Gawp" , "Stare stupidly" , "Stare with open mouth in wonder" .)