A trial arranged for a star (6)
I believe the answer is:
altair
'star' is the definition.
(Altair is a star in the constellation of Aquila)
'a trial arranged for a' is the wordplay.
'arranged' indicates anagramming the letters (letters rearranged).
'for' means one lot of letters go next to another (I've seen this in other clues).
'trial' is an anagram of 'ltair'.
'ltair' put after 'a' is 'ALTAIR'.
(Other definitions for altair that I've seen before include "Lariat (anag) - star in Aquila" , "First-magnitude star" , "A trial (anag) - star" , "Bright star; lariat (anag.)" , "Heavenly body" .)