With a butterfly/moth confusion and a hint of Alice’s Adventures in Wonderland,
the definition could be the one you’ve suggested.

Maybe some sort of clarification is just understood?
“down” probably means “underground” (for the creature you enter as your answer)

If ‘went down’ were to mean reduced or shortened, then “Christopher Robin went down” would be Chris, ‘with Alice’ would give Chris-Alice, and ‘say’ indicating a homophone gives CHRYSALIS.

This would leave the definition as “down probably meaning underground”, which I am still stumped on, but at least it gives a well constructed wordplay.

I like the idea of splitting “down probably meaning underground” into either “down” and “probabbly meaning underground”, or “down probably” and “meaning undergound”.

The latter would be the definition and the former would be part of word play.

There may be a usage of chrysalis that means ‘a hidden body’ i.e. ‘underground’, in the sense of sub-culture or resistance fighters. It is not in my Chambers, but some other dictionary may have it.

Possibly, as jmac suggests, this definition would be accentuated if Cinephile has confused a chrysalis (definitely suspended above ground) with a cocoon that may be buried underground, and which conveys a greater sense of ‘hidden’ in its meanings and usages.

I still can’t work out how “down” or “down probably” works though.

The question mark at the end of the quotation may be emphasising the word DOWN.
I think it can mean ‘in a comatose state’ after, say, an anaesthetic.
UNDERGROUND can mean ‘concealed and done in secret’.

Both of these seem to describe a CHRYSALIS quite well.
I’m not sure how to connect this with the noun in the solution though!

But I am as mystified as everyone else about how the clue works.