Basis sent coroner mad (11)
I believe the answer is:
cornerstone
'basis' is the definition.
(I know that basis can be written as cornerstone)
'sent coroner mad' is the wordplay.
'mad' indicates an anagram.
'sent'+'coroner'='sentcoroner'
'sentcoroner' anagrammed gives 'CORNERSTONE'.
(Other definitions for cornerstone that I've seen before include "Supporter" , "Something of prime importance" , "Mainstay" , "Key figure" .)