The most common and most likely answer to this crossword clue is the 6 letter word ERECTS
I've seen this crossword clue in The Daily Telegraph
E
R
E
C
T
S
Buddy explains!

The straight part
We know that "Assembles" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case assembles = erects.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 ERECTS mean?
ERECTS verb- To put up by the fitting together of materials or parts.
- To cause to stand up or out.
- To raise and place in an upright or perpendicular position; to set upright; to raise.
- To lift up; to elevate; to exalt; to magnify.
- To animate; to encourage; to cheer.
- To cast or draw up (a figure of the heavens, horoscope etc.).
- To set up as an assertion or consequence from premises, etc.
- To set up or establish; to found; to form; to institute.