Brightest star has finally dropped behind mountain (5)
I believe the answer is:
alpha
'brightest star' is the definition.
(I've seen this before)
'has finally dropped behind mountain' is the wordplay.
'finally dropped' means to remove the last letter.
'behind' says to put letters next to each other.
'mountain' becomes 'alp' (the Alps are a mountain range).
'has' with its last letter taken off is 'ha'.
'ha' put after 'alp' is 'ALPHA'.
(Other definitions for alpha that I've seen before include "star outshining the others" , "First in sequence" , "very good rating" , "Leading Greek character" , "Foreign character" .)