South African sheltering unknown pugilist (5)
I believe the answer is:
boxer
'pugilist' is the definition.
(synonyms)
'south african sheltering unknown' is the wordplay.
'south african' becomes 'boer' (Dutch settler in South Africa).
'sheltering' is an insertion indicator.
'unknown' becomes 'x' (algebra).
'boer' enclosing 'x' is 'BOXER'.
(Other definitions for boxer that I've seen before include "Breed of stocky short-haired dog" , "Dog - fighter" , "canine" , "Prizefighter" , "Person assisted by a second" .)