It's highly brilliant but not meteoric (5)
I believe the answer is:
comet
'highly brilliant but not meteoric' is the definition.
I can't tell whether this defines the answer.
'it's' is the wordplay.
I cannot really see how this works, but
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is found within the answer.
The remaining letters 'come' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for comet that I've seen before include "Tailed space traveller" , "Omen supposedly" , "Celestial body with luminous tail" , "body found in space" , "Heavenly body with a fiery tail" .)