A good outcome to queen's labour? Brilliant! (8)
I believe the answer is:
aglitter
'brilliant' is the definition.
'a good outcome to queen's labour?' is the wordplay.
'a good' becomes 'ag' (I've seen this in other clues).
'outcome to queen's labour?' becomes 'litter' (a female cat may give birth to a litter).
'ag'+'litter'='AGLITTER'
(Other definitions for aglitter that I've seen before include "Shiny" , "Sparkling with light" .)