The most common and most likely answer to this crossword clue is the 5 letter word NIDUS
I've seen this crossword clue in The Guardian
N
I
D
U
S
Buddy explains!

The straight part
We know that "Nest" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case nest = nidus.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 NIDUS mean?
NIDUS noun- An aggregate of neurons.
- A nest for insects or small animals.
- A place of infection in an organism.
- An origin (originating point) for a phenomenon.