The most common and most likely answer to this crossword clue is the 11 letter word PRESUPPOSED
I've seen this crossword clue in The Daily Telegraph
P
R
E
S
U
P
P
O
S
E
D
Buddy explains!
The straight part
We know that "taken for granted" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case taken for granted = presupposed.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 PRESUPPOSED mean?
PRESUPPOSED verb- To assume some truth without proof, usually for the purpose of reaching a conclusion based on that truth.