Top performer gets in a measure of luminaries (7)
I believe the answer is:
stellar
'top performer' is the definition.
Both the answer and definition are adjectives. Maybe you can see an association between them that I don't see?
'in a measure of luminaries' is the wordplay.
'in' is an insertion indicator.
'a measure' becomes 'sr' (sr is a kind of measure).
'of luminaries' becomes 'tella' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'sr' going around 'tella' is 'STELLAR'.
'gets' acts as a link.
Can you help me to learn more?
(Other definitions for stellar that I've seen before include "Starry" , "Wonderful" , "boss!" , "Top-notch" , "Excellent - shining bright?" .)