Brown will maybe lose out with a growl! (6)
I believe the answer is:
sorrel
'brown' is the definition.
(thesaurus)
'lose out with a growl' is the wordplay.
'out' indicates anagramming the letters (out can mean wrong or inaccurate).
'with' means one lot of letters goes inside another.
'a growl' becomes 'rr' (I can't explain this - if you can you should believe this answer much more).
'lose' anagrammed gives 'soel'.
'soel' enclosing 'rr' is 'SORREL'.
'will maybe' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for sorrel that I've seen before include "Weed - colour of horse" , "Light-chestnut horse" , "bay" , "Horse of reddish-brown colour" , "It provides flavour" .)