Aussie animal gets smart at rest (8)
I believe the answer is:
roosting
'at rest' is the definition.
(eg roosting birds are at rest)
'aussie animal gets smart' is the wordplay.
'aussie animal' becomes 'roo' (short for kangaroo).
'gets' is a charade indicator (letters next to each other).
'smart' becomes 'sting' (both can mean to burn or hurt).
'roo'+'sting'='ROOSTING'
(Other definitions for roosting that I've seen before include "Sitting like a bird" , "in place of sleep?" , "Trying to sleep" , "Of birds, settling for sleep" , "Perching, as fowl do" .)