The most common and most likely answer to this crossword clue is the 12 letter word BREAKTHROUGH
I've seen this crossword clue in The Guardian
B
R
E
A
K
T
H
R
O
U
G
H
More clues leading to the result BREAKTHROUGH
- Separate because of significant discovery
- Suggestion about king getting harsh in penetration of enemy lines
- Advance cast picked up after rest
- Penetrate enemy defences — a significant development
- Holiday finished with significant discovery
- Discovery holiday by river, falling in however
- Suggestion about thousand joining disorderly advance
- Advance from unsentimental banking personnel after time off
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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 BREAKTHROUGH mean?
BREAKTHROUGH noun- An advance through and past enemy lines.
- Any major progress; such as a great innovation or discovery that overcomes a significant obstacle.
- The penetration of the opposition defence
- The penetration of a separating wall or the remaining distance to an adjacent hollow (a crosscut in mining) or between two parts of a tunnel build from both ends; knockthrough.
- Characterized by major progress or overcoming some obstacle.