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' means one lot of letters goes inside another.
'50' becomes 'l' (Roman numeral).
'say' placed around 'l' is 'SLAY'.
(Other definitions for slay that I've seen before include "Butcher" , "Massacre" , "Fatally hit" , "Violently kill" , "Kill (literary)" .)