The most common and most likely answer to this crossword clue is the 10 letter word FORERUNNER
I've seen this crossword clue in The Daily Telegraph
F
O
R
E
R
U
N
N
E
R
Buddy explains!
The straight part
We know that "one that came first" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one that came first = forerunner.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 FORERUNNER mean?
FORERUNNER noun- A runner at the front or ahead.
- By extension, a non-competitor who leads out the competitors on to the circuit, or who runs/rides the course prior to competitor trials, usually testing or checking the way.
- A precursor or harbinger, a warning ahead.
- A forebear, an ancestor, a predecessor.
- A postage stamp used in the time before a region or area issues stamps of its own.