The most common and most likely answer to this crossword clue is the 6 letter word SODIUM
I've seen this crossword clue in The Daily Telegraph
S
O
D
I
U
M
More clues leading to the result SODIUM
- An element of singular dislike
- Metal, very thick, contains uranium
- Sum I do wrongly, although it’s elementary
- Incorporating cupped shape, extremely dull street lamps use this
- An element so unintelligent will be kept outside university
- Element of Society given blame
- Element of hatred under Stalin’s lead
- Element of personal antipathy against son
- Thus girl expresses hesitation for salt
- Singular repugnance for reactive element
Buddy explains!

The straight part
We know that "reactive element" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case reactive element = sodium.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 SODIUM mean?
SODIUM noun- The chemical element (symbol Na) with an atomic number of 11 and atomic weight of 22.98977. It is a soft, waxy, silvery, reactive alkali metal that is never found unbound in nature.