The most common and most likely answer to this crossword clue is the 11 letter word PREDECESSOR
I've seen this crossword clue in The Guardian
P
R
E
D
E
C
E
S
S
O
R
Buddy explains!

The straight part
We know that "the one before" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case the one before = predecessor.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 PREDECESSOR mean?
PREDECESSOR noun- One who precedes; one who has preceded another in any state, position, office, etc.; one whom another follows or comes after, in any office or position.
- A model or type of machinery or device which precedes the current one. Usually used to describe an earlier, outdated model.
- A vertex having a directed path to another vertex