Spain callously captures member of ancient empire (4)
I believe the answer is:
inca
'member of ancient empire' is the definition.
(member of the Inca Empire)
'spain callously captures' is the wordplay.
'captures' indicates a hidden word (the answer is trapped within the clue).
'INCA' can be found hidden inside 'spain callously'.
(Other definitions for inca that I've seen before include "Member of Andean people conquered by Spain" , "Pyramid builders" , "Member of old Peruvian Indian race" , "Pre-conquest South American people" , "Inhabitant of old Peru" .)