Losing first halves on return, Sweden and Norway displayed fatigue (6)
I believe the answer is:
yawned
'displayed fatigue' is the definition.
(yawning indicates tiredness)
I don't understand the rest of the clue.
(Other definitions for yawned that I've seen before include "Breathed deeply with mouth wide open" , "Opened mouth wide as expression of boredom" , "Demonstrated tiredness (or boredom!)" , "Showed tiredness or tedium" , "Gave visible sign of boredom - or tiredness" .)