The most common and most likely answer to this crossword clue is the 7 letter word DENDRON
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More clues leading to the result DENDRON
Buddy explains!

The straight part
We know that "cell extension" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case cell extension = dendron.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 DENDRON mean?
DENDRON noun- A slender projection of a nerve cell which conducts nerve impulses from a synapse to the body of the cell; a dendrite.
- A section of a dendrimer that includes the central atom or group.