Singular act of slaughter leads to row (5)
I believe the answer is:
scull
'row' is the definition.
(to scull is to row a boat with sculls)
'singular act of slaughter' is the wordplay.
'singular' becomes 's' (abbreviation in grammar).
'act of slaughter' becomes 'cull' (slaughter of animals).
's'+'cull'='SCULL'
'leads to' is the link.
(Other definitions for scull that I've seen before include "Small oar" , "Light rowing boat" , "A single oar" , "racing shell" , "Blade" .)