Underwear for dogs (6)
I believe the answer is:
boxers
This clue is a double definition.
'underwear' is the first definition.
(I know that boxers is a type of underwear)
'for dogs' is the second definition.
(boxer is a kind of dog)
(Other definitions for boxers that I've seen before include "Men's shorts for pugilists" , "Pugs; dogs" , "Dogs; packers" , "Dogs or pugilists" , "Craters" .)