Explosive device Aussie native kept in castle, perhaps (6)
I believe the answer is:
maroon
'explosive device' is the definition.
(maroon is a kind of explosive device)
'aussie native kept in castle perhaps' is the wordplay.
'aussie native' becomes 'roo' (short for kangaroo).
'kept in' indicates putting letters inside.
'castle perhaps' becomes 'man' (castle is a kind of man).
'roo' placed inside 'man' is 'MAROON'.
(Other definitions for maroon that I've seen before include "Leave stranded with little hope of rescue" , "Abandon (eg, on island)" , "Dark brownish-red" , "distress signal" , "Slaves' descendant" .)