The most common and most likely answer to this crossword clue is the 1 plus 6 plus 3 plus 3 letter word A BRIDGE TOO FAR
I've seen this crossword clue in The Guardian
A
B
R
I
D
G
E
T
O
O
F
A
R
More clues leading to the result A BRIDGE TOO FAR
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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.