The brightest star in the constellation Aquila (6)
I believe the answer is:
altair
'the brightest star in the constellation aquila' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for altair that I've seen before include "Bright star in Aquila" , "Member of Eagle family" , "Heavenly body" , "First-magnitude star" , "Bright star; lariat (anag.)" .)