Hold back, as a yawn (6)
I believe the answer is:
stifle
'hold back as a yawn' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for stifle that I've seen before include "Have difficulty breathing" , "Smother, suffocate" , "Quash" , "Joint in hind leg of horse" , "Repress, choke" .)