The most common and most likely answer to this crossword clue is the 8 letter word DIVAGATE
I've seen this crossword clue in The Guardian
D
I
V
A
G
A
T
E
More clues leading to the result DIVAGATE
Buddy explains!

The straight part
We know that "Rarely digress" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case rarely digress = divagate.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 DIVAGATE mean?
DIVAGATE verb- To wander about.
- To stray from a subject or theme.