The most common and most likely answer to this crossword clue is the 5 letter word ALLYL
I've seen this crossword clue in The Guardian
A
L
L
Y
L
Buddy explains!

The straight part
We know that "radical" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case radical = allyl.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 ALLYL mean?
ALLYL noun- (especially in combination) The univalent radical, CH2=CH-CH2-, existing especially in oils of garlic and mustard.