The most common and most likely answer to this crossword clue is the 3 plus 2 plus 5 letter word OUT TO LUNCH
I've seen this crossword clue in Financial Times
O
U
T
T
O
L
U
N
C
H
More clues leading to the result OUT TO LUNCH
Buddy explains!
The straight part
We know that "Cuckoo/dining away from nest?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case cuckoo/dining away from nest? = out to lunch.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.