The most common and most likely answer to this crossword clue is the 10 letter word ASSUMPTION
I've seen this crossword clue in The Daily Telegraph
A
S
S
U
M
P
T
I
O
N
More clues leading to the result ASSUMPTION
- Rees-Mogg finally abandoning common sense in support of fool’s unjustified claim
- Biblical event which is thought to be true
- Point out a small problem before making hypothesis
- What some Christians believe in – it’s taken for granted
- Taking for granted past in sumo wrestling
- Belief man is so put out
- Claim in Catholic feast
- … /08 from fool with initiative initially lost
Buddy explains!

The straight part
We know that "Claim/Catholic feast" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case claim/catholic feast = assumption.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 ASSUMPTION mean?
ASSUMPTION noun- The act of assuming, or taking to or upon oneself; the act of taking up or adopting.
- The act of taking for granted, or supposing a thing without proof; a supposition; an unwarrantable claim.
- The thing supposed; a postulate, or proposition assumed; a supposition.
- The minor or second proposition in a categorical syllogism.
- The taking of a person up into heaven.
- A festival in honor of the ascent of the Virgin Mary into heaven, celebrated on 15 August.
- Assumptio.