The most common and most likely answer to this crossword clue is the 4 letter word MRNA
I've seen this crossword clue in The Guardian
M
R
N
A
Buddy explains!
The straight part
We know that "In a cell, what carries information" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case in a cell, what carries information = mrna.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 MRNA mean?
MRNA noun- RNA that encodes and carries information from DNA during transcription to sites of protein synthesis to undergo translation in order to yield a protein.