The most common and most likely answer to this crossword clue is the 3 plus 1 plus 4 letter word GET A ROOM
I've seen this crossword clue in The Guardian
G
E
T
A
R
O
O
M
More clues leading to the result GET A ROOM
Buddy explains!

The straight part
We know that "What’s said when one’s due to be cuddled by future partner" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case what’s said when one’s due to be cuddled by future partner = get a room.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.