The most common and most likely answer to this crossword clue is the 8 letter word GRAPHENE
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The straight part
We know that "Form of 5" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case form of 5 = graphene.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 GRAPHENE mean?
GRAPHENE noun- Any polycyclic aromatic hydrocarbon having the structure of part of a layer of graphite.
- An arbitrarily large-scale, one-atom-thick layer of graphite, an allotrope of carbon, that has remarkable electric characteristics.