The most common and most likely answer to this crossword clue is the 6 letter word NUANCE
I've seen this crossword clue in The Guardian
N
U
A
N
C
E
More clues leading to the result NUANCE
- Subtlety shown by woman of faith taking over a church
- Subtle difference in tone as sister describes area close to church
- Trace sister pinching a case of champagne
- Refinement shown by sister touring a church
- Annoying person is lacking subtlety
- Nuisance is removed with subtlety
- Subtlety is absent in pest
- Religious figure retaining a church distinction
- Shade that makes little difference
- Introducing neat uniforms and nice cords with extra fine distinction
Buddy explains!

The straight part
We know that "subtlety" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case subtlety = nuance.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 NUANCE mean?
NUANCE noun- A minor distinction.
- Subtlety or fine detail.
- To apply a nuance to; to change or redefine in a subtle way.