Any recommendations for a bicycle backpack which can be easily clipped on a bicycle tray in the back?

Ortlieb bags seem pretty good in my experience. I have the backroller XL which is a pannier bag with a shoulder strap rather than a backpack. Plus they are made in Germany and not just a German company with outsourced production.

Can also vouch for Ortlieb. Back when I bike commuted to the office, I used an Ortlieb single pannier for my laptop and accessories. Was pretty convenient to clip my bag to the bike and not get my back too sweaty from riding with a backpack.

Yeah, as a former sweaty back commuter its a big improvement to not have a backpack on. Plus I get fewer holes in the back of my t-shirts now from friction between them and the backpack.

Although with the heatwaves right now I still get fairly sweaty, at least not having a backpack on makes it a bit better.

The arkel brand has some super well made panniers.

https://arkel.ca/collections/bicycle-panniers/products/bug-pannier-backpack

North St bags makes a pannier backpack also.

https://northstbags.com/

They are both extremely heavy duty with high quality hardware, burly fabric etc

I recommend buying a brompton bicycle.

• https://www.bromptontraveler.com/2013/09/backpack-on-brompton.html

The rack being super low means the distance between the rear rack and the saddle is almost the exact height of a good backpack. IMHO, it's the best way to carry a backpack on a bicycle

Edit: Brompton's small front O-bag is actually made by Ortlieb. I wouldn't recommend their bags as backpack replacements. They work great on bicycles, but not so much on your back (where the weight should actually be on your hips)

I really liked this one where the backpack straps and panier attachments are on opposite sides of the bag, the conversion is quicker (just flip the top flap over) and it's more comfortable as a backpack than others I've tried. It got me through 4 years of college (overloaded with textbooks biking ~8 mi each way) but definitely started to show some wear, probably not quite as durable as an ortlieb but a great bag for $99.

https://banjobrothers.com/collections/backpack-panniers-banjobrothers/products/backpack-pannier-banjo-brothers

Ditto, Ortlieb Backrollers are great! I'm amazed at how much can fit in them too, I can get a week's worth of groceries in a pair of them. (Some bulky things like a cereal box get strapped on the rack between them.)

They have a pretty solid backpack pannier as well

My favorite backpack brand is Osprey. They have a warranty that states, "we don't care if you bought it on 1972 (or something like that) or yesterday, we will fix it" which to me is awesome, better than a replacement. I've had great luck with their packs.

Any backpack with an internal frame works fine. These are usually long distance hiking packs, so they last a long time.