The most common and most likely answer to this crossword clue is the 4 plus 2 plus 5 letter word REST IN PEACE
I've seen this crossword clue in The Daily Telegraph
R
E
S
T
I
N
P
E
A
C
E
More clues leading to the result REST IN PEACE
Buddy explains!

The straight part
We know that "farewell words" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case farewell words = rest in peace.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.