The most common and most likely answer to this crossword clue is the 13 letter word PERFECTIONIST
I've seen this crossword clue in The Daily Telegraph
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More clues leading to the result PERFECTIONIST
- Person who's demanding to inspect fire training
- Stickler for niceties hurt in physical training
- One demands the best — often priciest — variety
- He wants things just so ropes etc fit in buggy
- One prefers things not so-so, but just so
- Pedant tense? No, it’s I, corrected
- Stickler for niceties breaking down in physical training
- Production of ice nets profit for pedant
- Proficient set excited teacher demanding the best?
Buddy explains!

The straight part
We know that "One prefers things not so-so, but just so" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one prefers things not so-so, but just so = perfectionist.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 PERFECTIONIST mean?
PERFECTIONIST noun- Someone who is unwilling to settle for anything that is not perfect or does not meet extremely high standards.
- Someone who thinks that religious or moral perfection can be attained in this life.
- One of the Bible Communists or Free-lovers, a small American sect founded by J. H. Noyes (1811-86), which settled at Oneida in 1848, holding that the gospel if accepted secures freedom from sin.
- Demanding perfection; being a perfectionist; imposing overly high standards.
- Inclined or related to perfectionism.