Crushing retort from second schoolmaster to Billy Bunter (7)
I believe the answer is:
squelch
'crushing retort' is the definition.
(squelch can mean a good argument or retort)
'second schoolmaster to billy bunter' is the wordplay.
'second' becomes 's' (common abbreviation).
'schoolmaster to billy bunter' becomes 'Quelch' (teacher in the Billy Bunter books).
's'+'quelch'='SQUELCH'
'from' acts as a link.
(Other definitions for squelch that I've seen before include "Noise of walking through wet mud" , "Sound of boots in mud" , "Sound of a boot in mud" , "Walk noisily through soft mud" , "silence" .)