Obscure brash Yankee about to take the lead (6)
I believe the answer is:
cloudy
'obscure' is the definition.
(I've seen this before)
'brash yankee about to take the lead' is the wordplay.
'brash' becomes 'loud' (I've seen this before).
'yankee' becomes 'y' (phonetic alphabet: alpha, bravo, charlie etc.).
'about' becomes 'c' (abbreviation for 'circa').
'to take the lead' is a charade indicator (letters next to each other) (some letters go in front of others).
'loud'+'y'='loudy'
'loudy' after 'c' is 'CLOUDY'.
(Other definitions for cloudy that I've seen before include "Not transparent" , "(Of a liquid) not clear" , "Dull, overcast" , "Obscure" , "Murky or nebulous" .)