Good copy, but yawn! (4)
I believe the answer is:
gape
'yawn' is the definition.
('gape' can be a synonym of 'yawn')
'good copy' is the wordplay.
'good' becomes 'g' (abbreviation).
'copy' becomes 'ape' ('ape' can be a synonym of 'copy').
'g'+'ape'='GAPE'
'but' acts as a link.
(Other definitions for gape that I've seen before include "Look in amazement or stupidity" , "Failure to meet" , "Peer" , "Gawp" , "Yawn" .)