The most common and most likely answer to this crossword clue is the 9 letter word LAUNCHING
I've seen this crossword clue in The Daily Telegraph
L
A
U
N
C
H
I
N
G
Buddy explains!
The straight part
We know that "useful start" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case useful start = launching.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 LAUNCHING mean?
LAUNCHING verb- To throw (a projectile such as a lance, dart or ball); to hurl; to propel with force.
- To pierce with, or as with, a lance.
- To cause (a vessel) to move or slide from the land or a larger vessel into the water; to set afloat.
- To cause (a rocket, balloon, etc., or the payload thereof) to begin its flight upward from the ground.
- To send out; to start (someone) on a mission or project; to give a start to (something); to put in operation
- To start (a program or feature); to execute or bring into operation.
- To release; to put onto the market for sale
- Of a ship, rocket, balloon, etc.: to depart on a voyage; to take off.
- (often with out) To move with force and swiftness like a sliding from the stocks into the water; to plunge; to begin.
- (of a program) To start to operate.
- The act by which something is launched; a launch.