The most common and most likely answer to this crossword clue is the 6 letter word OPTANT
I've seen this crossword clue in Financial Times
O
P
T
A
N
T
Buddy explains!

The straight part
We know that "I choose" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case i choose = optant.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 OPTANT mean?
OPTANT noun- A person who lives in a region undergoing a change of sovereignty and thus may choose between retaining their old citizenship or opting for the citizenship of the new sovereignty.
- A person who opts into, out of, or for something.