The most common and most likely answer to this crossword clue is the 6 letter word LOGGER
I've seen this crossword clue in The Daily Telegraph
L
O
G
G
E
R
More clues leading to the result LOGGER
Buddy explains!
The straight part
We know that "Maybe a feller who keeps a diary" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case maybe a feller who keeps a diary = logger.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 LOGGER mean?
LOGGER noun- A worker whose occupation is to harvest trees.
- That which logs, such as a computer program to keep track of events.