Show ennui (4)
I believe the answer is:
yawn
'show ennui' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for yawn that I've seen before include "Sleepy gape" , "Mouth-stretching" , "Involuntary sign of tiredness or boredom" , "Boring event" , "Show tiredness/boredom" .)