Brown may be a loser outright (6)
I believe the answer is:
sorrel
'brown' is the definition.
(thesaurus)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for sorrel that I've seen before include "Weed - colour of horse" , "Perennial herb" , "bay" , "It provides flavour" , "Horse of reddish-brown colour" .)