Men's underpants (6)
(Other definitions for boxers that I've seen before include "in the ring with shorts" , "Men's shorts for pugilists" , "Dogs; packers" , "Pugs; dogs" , "Pugilists or men's underwear" .)
'men's underpants' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for boxer shorts that I've seen before include "pants" , "Male underwear" , "Men's underpants" , "Men's underwear" .)