Reduced the population by selective slaughter (6)
I believe the answer is:
culled
'reduced the population by selective slaughter' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for culled that I've seen before include "Reduced the population by selective slaughter" , "Picked out and killed for the good of the herd" , "Picked out and killed surplus animals from herd" , "Of deer, killed off surplus numbers" , "Of deer, killed surplus animals" .)