[[:a] ]

If I have the point on the second line after the space and before the close bracket then lispy-backward-kill-word will create

[[:]

another example

[{:a :b} ]

gives

[{:a :]

I'm on the latest emacs in master c24766c4d59bc7d9f583f299b1558e2356fba933 from Tue Mar 23 and the latest from lispy 38a7df4 Thu Jan 21.