Egghead let client stew (9)
I believe the answer is:
intellect
'egghead' is the definition.
(thesaurus)
'let client stew' is the wordplay.
'stew' indicates an anagram.
'let'+'client'='letclient'
'letclient' with letters rearranged gives 'INTELLECT'.
(Other definitions for intellect that I've seen before include "Brain" , "Cognitive faculty" , "Person with brilliant mind" , "Power of the mind" , "Reason" .)