The most common and most likely answer to this crossword clue is the 7 letter word GAMETES
I've seen this crossword clue in The Guardian
G
A
M
E
T
E
S
More clues leading to the result GAMETES
Buddy explains!

The straight part
We know that "cells" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case cells = gametes.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 GAMETES mean?
GAMETES noun- A reproductive cell (sperm in males or eggs in females), having only half of a complete set of chromosomes.