The most common and most likely answer to this crossword clue is the 9 letter word INITIALLY
I've seen this crossword clue in The Guardian
I
N
I
T
I
A
L
L
Y
More clues leading to the result INITIALLY
- Trendy Italian, one person on the same side to begin with
- For beginners, how to read Faust, usually not for fun?
- Ignoring the odds, Kiwi nurses fool friend at first
- My reason for being in league to begin with
- First XI’s carrying idiot partner
- First of all, I’m not intending to idle about like last year. That’s to begin with…
- Ground in Italy lit with no end of excitement in the beginning
- At first, during sex, one is accompanied by partner
Buddy explains!

The straight part
We know that "to begin with" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case to begin with = initially.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 INITIALLY mean?
INITIALLY adverb- At the beginning.