Scoff when leader is knocked out in preliminary round (3)
I believe the answer is:
eat
'scoff' is the definition.
(to scoff something is to eat it quickly)
'when leader is knocked out in preliminary round' is the wordplay.
'when leader is knocked out in' means to remove the first letter.
'preliminary round' becomes 'heat' (I've seen this before).
'heat' with its initial letter taken off is 'EAT'.
(Other definitions for eat that I've seen before include "Bite, chew and swallow" , "tolerate in diet" , "Be corrosive" , "Scoff" , "and 27: Be humiliated" .)