The most common and most likely answer to this crossword clue is the 10 letter word HARDHEADED
I've seen this crossword clue in The Times
H
A
R
D
H
E
A
D
E
D
Buddy explains!
The straight part
We know that "One who is this still needs a crash helmet, being unsentimental" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one who is this still needs a crash helmet, being unsentimental = hardheaded.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 HARDHEADED mean?
HARDHEADED adjective- Stubborn; wilful.
- Realistic; pragmatic.