The most common and most likely answer to this crossword clue is the 11 letter word PERMANENTLY
I've seen this crossword clue in Financial Times
P
E
R
M
A
N
E
N
T
L
Y
More clues leading to the result PERMANENTLY
Buddy explains!
The straight part
We know that "forever" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case forever = permanently.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 PERMANENTLY mean?
PERMANENTLY adverb- In a permanent manner; lastingly.
- Forever.