Southern Canadian rock group on downward trajectory (5)
I believe the answer is:
scree
'rock group on downward trajectory' is the definition.
(scree means loose stones on a slope)
'southern canadian' is the wordplay.
'southern' becomes 's' (abbreviation).
'canadian' becomes 'Cree' (the Cree are indigenous to Canada).
's'+'cree'='SCREE'
(Other definitions for scree that I've seen before include "Debris at the foot of a cliff" , "Loose stones on mountain slope" , "Safeguard" , "Loose shifting stones" , "Loose stones around a mountain" .)