Who would happily make you smart (6)
I believe the answer is:
sadist
'you smart' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'who would happily' is the wordplay.
I cannot quite see how this works, but
'would' could be 'd' (I've seen this in other clues) and 'd' is found within the answer.
The remaining letters 'saist' is a valid word which might be clued in a way I don't see.
This may be the basis of clue (or it may be nonsense).
'make' acts as a link.
(Other definitions for sadist that I've seen before include "Person who enjoys inflicting pain on others" , "One who takes delight in cruelty" , "Rub out, obliterate" , "One enjoying hurting others" , "One takes pleasure in inflicting pain" .)