The most common and most likely answer to this crossword clue is the 6 plus 8 letter word NORMAN CONQUEST
I've seen this crossword clue in The Guardian
N
O
R
M
A
N
C
O
N
Q
U
E
S
T
More clues leading to the result NORMAN CONQUEST
- Sandhurst NCO involved in what’s not a search for military success
- Standard article by Tory, with mission to show England overrun by foreigners
- Wisdom to learn, seeking outcome of immigration from France
- & 15d William’s acquisition — Quran — connotes start of Mohammed’s movement
- Invasion and occupation: boy on mission to claim crown, primarily
Buddy explains!
The straight part
We know that "England overrun by foreigners" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case england overrun by foreigners = norman conquest.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.