Show boredom (4)
I believe the answer is:
yawn
'show boredom' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for yawn that I've seen before include "Show one is tired or bored" , "Intake of breath through a wide-open mouth" , "Show tiredness/boredom" , "Tedious thing" , "Oscitate" .)