I've googled, and the consensus seems to be that it's OK to use new mown
grass clippings to mulch beds.

To add to the "wisdom" of the Net, does anyone here have practical
experience of doing this?

The situation is a new-made perennials border, where the soil is made up
of double-dug clayey soil that had been under turf, mixed in with a
certain amount of soil conditioner, and old rotted down turf (i.e. dry
granular soil). The bed seems to want "body", at present, and dries out
too readily imho in the wind (and rare sun).

Cheers
John