The most common and most likely answer to this crossword clue is the 6 letter word ZEROTH
Z
E
R
O
T
H
More clues leading to the result ZEROTH
Buddy explains!

The straight part
We know that "Before the first" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case before the first = zeroth.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 ZEROTH mean?
ZEROTH adjective- In the initial position in a sequence whose elements are numbered starting at zero; the ordinal number corresponding to zero.
- Corresponding to a position preceding the first.