Knock out the best, boxing man I managed (9)
I believe the answer is:
eliminate
'knock out' is the definition.
(I've seen this before)
'the best boxing man i managed' is the wordplay.
'the best' becomes 'elite' (I've seen this before**).
'boxing' indicates putting letters inside ('box' can mean contain, as in to put in a box).
'managed' indicates anagramming the letters.
'man'+'i'='mani'
'mani' anagrammed gives 'mina'.
'elite' placed around 'mina' is 'ELIMINATE'.
(Other definitions for eliminate that I've seen before include "Do away with, get rid of" , "Do away with, remove" , "Knock out" , "Wipe out" , "put out" .)