A trial upset one very bright star (6)
I believe the answer is:
altair
'bright star' is the definition.
(I've seen this before)
'a trial upset' is the wordplay.
'upset' is an anagram indicator.
'trial' anagrammed gives 'ltair'.
'a'+'ltair'='ALTAIR'
'one very' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for altair that I've seen before include "Bright star; lariat (anag.)" , "First-magnitude star" , "Lariat (anag) - star in Aquila" , "Member of Eagle family" , "Bright star in Aquila" .)