Good sport suppressing a yawn (4)
I believe the answer is:
gape
'yawn' is the definition.
(synonyms)
'good sport suppressing a' is the wordplay.
'good' becomes 'g' (abbreviation).
'sport' becomes 'PE' (physical education).
'suppressing' is an insertion indicator.
'g'+'pe'='gpe'
'gpe' enclosing 'a' is 'GAPE'.
(Other definitions for gape that I've seen before include "Peer" , "Stare with open mouth in wonder" , "watch in wonder" , "Look with mouth wide open" , "Stare at page" .)