Kill, say, about fifty (4)
I believe the answer is:
slay
'kill' is the definition.
(slaying is a kind of killing)
'say about fifty' is the wordplay.
'about' means one lot of letters goes inside another.
'fifty' becomes 'l' ('L' can be a synonym of 'fifty').
'say' enclosing 'l' is 'SLAY'.
(Other definitions for slay that I've seen before include "Fatally hit" , "Kill, murder" , "Kill intentionally" , "Violently kill" , "Kill (literary)" .)