The most common and most likely answer to this crossword clue is the 6 letter word BITMAP
B
I
T
M
A
P
More clues leading to the result BITMAP
Buddy explains!

The straight part
We know that "format" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case format = bitmap.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 BITMAP mean?
BITMAP noun- A series of bits that represents a rasterized graphic image, each pixel being represented as a group of bits.