This boxer is chicken (6)
I believe the answer is:
bantam
This clue is a double definition.
'this boxer' is the first definition.
(bantamweight is a weight class in boxing)
'is chicken' is the second definition.
(I have seen 'Small chicken' mean 'bantam' so perhaps 'chicken' could also mean 'bantam')
(Other definitions for bantam that I've seen before include "Small chicken and light boxer" , "Supplier of eggs" , "Little clucker" , "weight of a pug" , "poultry" .)