Yawn, opening face at last (4)
I believe the answer is:
gape
'yawn' is the definition.
('gape' can be a synonym of 'yawn')
'opening face at last' is the wordplay.
'opening' becomes 'gap' (gap is a kind of opening**).
'at last' suggests the final letters.
The last letter of 'face' is 'e'.
'gap'+'e'='GAPE'
(Other definitions for gape that I've seen before include "Open-mouthed stare" , "Appear amazed" , "wide opening" , "Failure to meet" , "Stare with open mouth in wonder" .)