Anyone know what this is?

I suspect that isn't an off-the-shelf clip, but one manufactured specifically for that light fitting.

So I think you'll need to go back to the manufacturer.

(Have a gander on eBay for second-hand units - you might pick up a clip that way?)

 

Advice - change your florescents for led's

 

Agree,good advice, led's give a better light,which is brighter, cool running & cheaper to use.