The most common and most likely answer to this crossword clue is the 5 letter word GO-TO
I've seen this crossword clue in The Guardian
G
O
-
T
O
More clues leading to the result GO-TO
Buddy explains!

The straight part
We know that "So useful a person" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case so useful a person = go-to.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 GO-TO mean?
GO-TO noun- A person or thing that is one's choice to go to (for some purpose).
- Desired; desirable; of choice.
- Reliable; likely to perform in difficult circumstances.