The most common and most likely answer to this crossword clue is the 6 letter word SADIST
I've seen this crossword clue in The Guardian
S
A
D
I
S
T
More clues leading to the result SADIST
- Torturer is beginning to tremble after feeling sorrow
- It is time to arrest duke, a cruel man
- Seen in Jaws, a distressing monster
- Miserable meeting first vicious man
- Who would happily make you smart
- ‘Savage‘!? Sorry, it’s ‘unruly’
- Brute down first
- Brute causes a disturbance internally
- Pious type caging a wicked god, the brute
- One deriving perverse pleasure in art making statement about depth
Buddy explains!

The straight part
We know that "Who would happily make you smart" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case who would happily make you smart = sadist.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does SADIST mean?
SADIST noun- One who derives pleasure through cruelty or pain to others.