Small hen's killer (6)
I believe the answer is:
slayer
'killer' is the definition.
(I've seen this before)
'small hen's' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'hen' becomes 'layer' (I've seen this before).
's'+'layer'='SLAYER'
(Other definitions for slayer that I've seen before include "Violent killer" , "Butcher" , "Relays to killer" , "Assassin" , "Violent murderer" .)