The most common and most likely answer to this crossword clue is the 6 plus 8 letter word PIECED TOGETHER
I've seen this crossword clue in The Daily Telegraph
P
I
E
C
E
D
T
O
G
E
T
H
E
R
Buddy explains!
The straight part
We know that "How a jigsaw must be worked out" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case how a jigsaw must be worked out = pieced together.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.