The most common and most likely answer to this crossword clue is the 5 letter word SLICK
I've seen this crossword clue in The Daily Telegraph
S
L
I
C
K
More clues leading to the result SLICK
Buddy explains!

The straight part
We know that "Smart" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case smart = slick.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 SLICK mean?
SLICK noun- A covering of liquid, particularly oil.
- Someone who is clever and untrustworthy.
- A tool used to make something smooth or even.
- A tire with a smooth surface instead of a tread pattern, often used in auto racing.
- A helicopter.
- A camera-ready image to be used by a printer. The "slick" is photographed to produce a negative image which is then used to burn a positive offset plate or other printing device.
- A wide paring chisel used in joinery.
- To make slick.
- Slippery or smooth due to a covering of liquid; often used to describe appearances.
- Appearing expensive or sophisticated.
- Superficially convincing but actually untrustworthy.
- (often used sarcastically) Clever, making an apparently hard task easy.
- Extraordinarily great or special.
- Sleek; smooth
- The finer portion of crushed ore, as of gold, lead, or tin, separated by the water in certain wet processes.