Brilliance client somehow overcomes constraint; overturns (9)
I believe the answer is:
intellect
'brilliance' is the definition.
(thesaurus)
'client somehow overcomes constraint overturns' is the wordplay.
'somehow' is an anagram indicator.
'overcomes' is an insertion indicator.
'constraint overturns' becomes 'tel' (I can't justify this - if you can you should believe this answer much more).
'client' anagrammed gives 'inlect'.
'inlect' going around 'tel' is 'INTELLECT'.
Can you help me to learn more?
(Other definitions for intellect that I've seen before include "Faculty of reasoning" , "Mental powers; clever person" , "Cognitive faculty" , "Brain" , "Power of the mind" .)