Heartless agent in South African hat (6)
I believe the answer is:
boater
'hat' is the definition.
(I know that boater is a type of hat)
'heartless agent in south african' is the wordplay.
'heartless' suggests removing the centre.
'in' indicates putting letters inside.
'south african' becomes 'boer' (Dutch settler in South Africa).
'agent' with its centre taken out is 'at'.
'at' put within 'boer' is 'BOATER'.
(Other definitions for boater that I've seen before include "Flat-topped straw hat" , "Hat style" , "(Of exercise) improving oxygen intake" , "sailor, perhaps" , "one going in the junk?" .)