Act sleepy (4)
I believe the answer is:
yawn
'act sleepy' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for yawn that I've seen before include "Show one is tired or bored" , "Gape in tiredness" , "Expression of tiredness" , "Involuntary sign of tiredness or boredom" , "Show boredom" .)