The most common and most likely answer to this crossword clue is the 6 letter word UNROLL
U
N
R
O
L
L
Buddy explains!
The straight part
We know that "Flatten out" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case flatten out = unroll.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 UNROLL mean?
UNROLL verb- To straighten something that has been rolled, twisted or curled.
- To emerge, be revealed or become apparent; to unfold.
- To replace (a loop in a program) with a repetitive sequence of the individual instructions that the loop would carry out, sometimes used as an optimization.