The most common and most likely answer to this crossword clue is the 11 letter word EASTÂ ANGLIA
I've seen this crossword clue in The Guardian
E
A
S
T
Â
A
N
G
L
I
A
Buddy explains!

The straight part
We know that "part of Britain" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case part of britain = east anglia.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.