The most common and most likely answer to this crossword clue is the 6 letter word SOLIDS
S
O
L
I
D
S
Buddy explains!
The straight part
We know that "must be three-dimensional figures" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case must be three-dimensional figures = solids.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 SOLIDS mean?
SOLIDS noun- A substance in the fundamental state of matter that retains its size and shape without need of a container (as opposed to a liquid or gas).
- A three-dimensional figure (as opposed to a surface, an area, or a curve).
- A favor.
- An article of clothing which is of a single color throughout.
- (in the plural) Food which is not liquid-based.