The most common and most likely answer to this crossword clue is the 11 letter word CALIFORNIUM
I've seen this crossword clue in The Guardian
C
A
L
I
F
O
R
N
I
U
M
Buddy explains!

The straight part
We know that "Radioactive element" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case radioactive element = californium.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 CALIFORNIUM mean?
CALIFORNIUM noun- A transuranic chemical element (symbol Cf) with an atomic number of 98.