South African holding unknown dog (5)
I believe the answer is:
boxer
'dog' is the definition.
(I know that Boxer is a type of dog)
'south african holding unknown' is the wordplay.
'south african' becomes 'boer' (Dutch settler in South Africa).
'holding' indicates putting letters inside.
'unknown' becomes 'x' (algebra).
'boer' enclosing 'x' is 'BOXER'.
(Other definitions for boxer that I've seen before include "canine" , "Pug - dog" , "Dog - fighter" , "one that's accustomed to fighting" , "Pugilist - dog" .)