Show fatigue (3)
(Other definitions for sag that I've seen before include "Slump" , "Sink under weight" , "Hang down loosely" , "Drop" , "Subsidence" .)
'show fatigue' 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 "Show boredom" , "Show one is tired or bored" , "Sleepy gape" , "Expression of tiredness" , "Intake of breath through a wide-open mouth" .)