The most common and most likely answer to this crossword clue is the 5 letter word AXIOM
I've seen this crossword clue in The Guardian
A
X
I
O
M
More clues leading to the result AXIOM
- Accepted statement from a football team over millions
- A team order that’s generally accepted
- Man after a kiss in principle
- A sign of treasure on island in Irish sea? One assumes it is true
- Man with a cross to bear: that’s self-evident
- Saw a cross on an island
- Saw American chopper land in Irish Sea
- Robotic mind honoured fundamental law
- Gnome across in front of Douglas’s place
- Taxis come endlessly in principle
Buddy explains!

The straight part
We know that "It’s taken for granted" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case it’s taken for granted = axiom.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 AXIOM mean?
AXIOM noun- A seemingly self-evident or necessary truth which is based on assumption; a principle or proposition which cannot actually be proved or disproved.
- (proof theory) A fundamental assumption that serves as a basis for deduction of theorems; a postulate (sometimes distinguished from postulates as being universally applicable, whereas postulates are particular to a certain science or context).
- An established principle in some artistic practice or science that is universally received.