The most common and most likely answer to this crossword clue is the 6 letter word CLEAVE
I've seen this crossword clue in The Times
C
L
E
A
V
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Buddy explains!
The straight part
We know that "Fissure" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fissure = cleave.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 CLEAVE mean?
CLEAVE noun- Flat, smooth surface produced by cleavage, or any similar surface produced by similar techniques, as in glass.
- To split or sever something with, or as if with, a sharp instrument.
- To break a single crystal (such as a gemstone or semiconductor wafer) along one of its more symmetrical crystallographic planes (often by impact), forming facets on the resulting pieces.
- To make or accomplish by or as if by cutting.
- To split (a complex molecule) into simpler molecules.
- To split.
- Of a crystal, to split along a natural plane of division.
- To cling, adhere or stick fast to something; used with to or unto.