The most common and most likely answer to this crossword clue is the 8 letter word HANDIEST
I've seen this crossword clue in Financial Times
H
A
N
D
I
E
S
T
Buddy explains!
The straight part
We know that "It would be most useful" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case it would be most useful = handiest.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 HANDIEST mean?
HANDIEST adjective- Easy to use, useful.
- Nearby, within reach.
- Of a person: dexterous, skilful.
- Physically violent; tending to use one's fists.
- Of a freight ship: having a small cargo capacity (less than 40,000 DWT); belonging to the handysize class.