The most common and most likely answer to this crossword clue is the 9 letter word MEGAHERTZ
I've seen this crossword clue in The Times
M
E
G
A
H
E
R
T
Z
Buddy explains!
The straight part
We know that "Radio unit" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case radio unit = megahertz.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 MEGAHERTZ mean?
MEGAHERTZ noun- A unit equal to one million cycles per second.