Being somewhat somnolent it yawned (6)
I believe the answer is:
entity
'being' is the definition.
(I've seen this in other clues)
'somewhat somnolent it yawned' is the wordplay.
'somewhat' indicates a hidden word.
'ENTITY' is hidden in the letters of 'somnolent it yawned'.
(Other definitions for entity that I've seen before include "Something existing in its own right" , "Individual being" , "Essence, existence" , "A thing with distinct and independent existence" , "Presence" .)