Brightest star principally associated with sacred river (5)
I believe the answer is:
alpha
'brightest star' is the definition.
(I've seen this before)
'principally associated with sacred river' is the wordplay.
I cannot quite see how this works, but
'river' could be 'alph' (I've seen this before) and 'alph' is present in the answer.
A single letter 'a' remains which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for alpha that I've seen before include "star outshining the others" , "piece of Greek writing" , "letter written in Greek" , "First (class)" , "high mark" .)