The most common and most likely answer to this crossword clue is the 7 letter word TIDIEST
I've seen this crossword clue in Independent
T
I
D
I
E
S
T
More clues leading to the result TIDIEST
- Most neat, one passes on dry clothes
- Goes out in clothing of singer: most neat!
- The largest bird withers inwardly
- One passes on dry clothes, smarter than anything
- Smartest independent investigator, inspired by trial
- One girl in exam is neater than all the rest
- Most trim little flier expires trapped inside
- With maximal organisation, temperature falls in it
- Most efficient time — that is, about 1
- Time I put in that is optimally arranged
Buddy explains!

The straight part
We know that "Smartest" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case smartest = tidiest.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 TIDIEST mean?
TIDIEST adjective- Arranged neatly and in order.
- Not messy; neat and controlled.
- Satisfactory; comfortable.
- Generous, considerable.
- In good time; at the right time; timely; seasonable; opportune; favourable; fit; suitable.
- Brave; smart; skillful; fine; good.
- Appropriate or suitable as regards occasion, circumstances, arrangement, or order.