The most common and most likely answer to this crossword clue is the 8 letter word INDWELLS
I've seen this crossword clue in The Guardian
I
N
D
W
E
L
L
S
Buddy explains!
The straight part
We know that "Remains for ever" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case remains for ever = indwells.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 INDWELLS mean?
INDWELLS verb- To exist within, especially as a spirit or driving force.