The most common and most likely answer to this crossword clue is the 7 letter word SUBTLER
I've seen this crossword clue in The Sunday Telegraph
S
U
B
T
L
E
R
Buddy explains!

The straight part
We know that "It’s not so obvious" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case it’s not so obvious = subtler.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 SUBTLER mean?
SUBTLER adjective- Hard to grasp; not obvious or easily understood; barely noticeable.
- (of a thing) Cleverly contrived.
- (of a person or animal) Cunning, skillful.
- Insidious.
- Tenuous; rarefied; of low density or thin consistency.
- Refined; exquisite.