Building a little runabout from spare bits and bobs. Single chainring upfront with no front derailleur but 9 speed cassette with derailluer on the rear.

Am I gonna need some kind chain device/tensioner to stop the chain jumping off the front ring and throwing me under a car, or you think I'll get away with it? Any recommendations if I do need a device?

Cheers.

Should be OK, chains don't normally jump off chainrings on a 3×9 set up so why should they on a 1×9?

Use an old front mech with the limit screws adjusted if you want a cheap option for a chain device.

I didnt need a chaindevice when using a singlespeed, unramped chainring (E13 guide ring, excellent), but as i'm now using a Rotor one (ramped) then i've had to use a Rohloff one. Paul comp make a nice one as well, or MRP do a specific one as well.

I also keep the chain as short and taught as possible, as this also helps.

We sell the mrp chain guides – very nice.
But on my 1×9 salsa soft tail I have not yet needed one. Not yet. But I will be fitting one now we have had another delivery.
An old mech can also be used if you twiddle the + & – screws.
Hope this helps
c