Inscrutable character of endlessly kind mother (6)
I believe the answer is:
enigma
'inscrutable character' is the definition.
(a mysterious person might be called an enigma)
'endlessly kind mother' is the wordplay.
'endlessly' says to take the centre (the ends of the words are removed).
'kind' becomes 'benign' (benign can mean kind or generous).
'mother' becomes 'ma' (Ma is another word for mother or mum).
The middle letters of 'benign' are 'enig'.
'enig'+'ma'='ENIGMA'
'of' acts as a link.
(Other definitions for enigma that I've seen before include "It's mysterious like gamine" , "WW2 code" , "Coding device" , "Puzzle for gaming" , "'Mystery, riddle (6)'" .)