Brewing cartels' brilliant red (7)
I believe the answer is:
scarlet
'brilliant red' is the definition.
(I've seen this before)
'brewing cartels'' is the wordplay.
'brewing' is an anagram indicator.
'cartels' with letters rearranged gives 'SCARLET'.
(Other definitions for scarlet that I've seen before include "Cartels (anag.)" , "Red colour of clarets" , "Shade of red" , "Brilliant red colour" , "Crimson" .)