The most common and most likely answer to this crossword clue is the 4 plus 5 plus 4 letter word MAKE FIRST BASE
M
A
K
E
F
I
R
S
T
B
A
S
E
Buddy explains!
The straight part
We know that "to complete stage one" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case to complete stage one = make first base.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.