The most common and most likely answer to this crossword clue is the 6 letter word FORMED
I've seen this crossword clue in The Sunday Telegraph
F
O
R
M
E
D
Buddy explains!
The straight part
We know that "Assembled" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case assembled = formed.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 FORMED mean?
FORMED verb- To assume (a certain shape or visible structure).
- To give (a shape or visible structure) to a thing or person.
- To take shape.
- To put together or bring into being; assemble.
- To create (a word) by inflection or derivation.
- To constitute, to compose, to make up.
- To mould or model by instruction or discipline.
- To provide (a hare) with a form.
- To treat (plates) to prepare them for introduction into a storage battery, causing one plate to be composed more or less of spongy lead, and the other of lead peroxide. This was formerly done by repeated slow alternations of the charging current, but later the plates or grids were coated or filled, one with a paste of red lead and the other with litharge, introduced into the cell, and formed by a direct charging current.