Brightest star in the constellation Aquila (6)
I believe the answer is:
altair
'brightest star in the constellation aquila' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for altair that I've seen before include "A trial (anag) - star" , "Bright star in Aquila" , "Lariat (anag) - star in Aquila" , "Bright star; lariat (anag.)" , "First-magnitude star" .)