The most common and most likely answer to this crossword clue is the 8 letter word RECLUSES
I've seen this crossword clue in The Guardian
R
E
C
L
U
S
E
S
More clues leading to the result RECLUSES
Buddy explains!

The straight part
We know that "they’re seldom spotted" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case they’re seldom spotted = recluses.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 RECLUSES mean?
RECLUSES noun- A person who lives in self-imposed isolation or seclusion from the world, especially for religious purposes; a hermit
- The place where a recluse dwells; a place of isolation or seclusion
- A brown recluse spider