The most common and most likely answer to this crossword clue is the 5 plus 2 plus 1 plus 9 letter word PLAIN AS A PIKESTAFF
I've seen this crossword clue in The Guardian
P
L
A
I
N
A
S
A
P
I
K
E
S
T
A
F
F
Buddy explains!
The straight part
We know that "It’s quite obvious" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case it’s quite obvious = plain as a pikestaff.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.