The most common and most likely answer to this crossword clue is the 11 letter word INTELLIGENT
I've seen this crossword clue in The Times
I
N
T
E
L
L
I
G
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T
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 = intelligent.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 INTELLIGENT mean?
INTELLIGENT adjective- Of high or especially quick cognitive capacity, bright.
- Well thought-out, well considered.
- Characterized by thoughtful interaction.
- Having the same level of brain power as mankind.
- Having an environment-sensing automatically-invoked built-in computer capability.