Winnow (4)
'winnow' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for cull that I've seen before include "Select to kill to lower numbers" , "Gather selectively" , "Choose the best from" , "Proportionate kill" , "Remove from herd" .)
(Other definitions for pare that I've seen before include "Trim by cutting" , "Shave or peel" , "Whittle" , "Remove the skin from" , "Shave, say, pencil" .)