The most common and most likely answer to this crossword clue is the 4 plus 8 letter word DEAD GIVEAWAY
I've seen this crossword clue in The Guardian
D
E
A
D
G
I
V
E
A
W
A
Y
Buddy explains!
The straight part
We know that "a pretty obvious clue?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a pretty obvious clue? = dead giveaway.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.