Brown will maybe lose out with a growl! (6)
I believe the answer is:
sorrel
'brown' is the definition.
(thesaurus)
'maybe lose out with a growl' is the wordplay.
'maybe' indicates an anagram.
'out' means to remove the last letter.
'with' is an insertion indicator.
'a growl' becomes 'rre' (I can't explain this - if you can you should believe this answer much more).
'lose' with its last letter taken off is 'los'.
'los' is an anagram of 'sol'.
'sol' enclosing 'rre' is 'SORREL'.
'will' is the link.
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" .)