Kill, say, about 50 (4)
I believe the answer is:
slay
'kill' is the definition.
(slaying is a kind of killing)
'say about 50' is the wordplay.
'about' indicates putting letters inside.
'50' becomes 'l' (Roman numeral).
'say' going around 'l' is 'SLAY'.
(Other definitions for slay that I've seen before include "Butcher" , "Massacre" , "Fatally hit" , "Violently kill" , "Kill (literary)" .)