Nine speed derailleur on 10 speed cassette Will combination work? I have 10 speed shifters, 10 speed chain, 10 speed cassette and all components are Shimano.

The reason I ask is because I am new to cycling and I am having trouble indexing my gears. Gears skip and I get some catching. It just might be not indexing properly or it might be the combination.

From a comment: 


  It’s a road bike but decided to go flat bar initially from a comfort aspect, hence the reason for the Tiagra flatbar shifters - 10 speed. All components are new, frame is second hand, hanger is straight from what I can tell. Cassette is 10 speed Ultegra, derailleur 105 9 speed, chain is a 10 speed SRAM. It’s a mix and I would have gone all 105 10 speed but I bought it aftermarket from a bike shop, and I am only making sense of the components now.


EDIT: Take delivery of the Tiagra RD 4700 tomorrow.Will let everyone know how it goes.
Assuming you have Shimano components, it depends if you have an MTB or road groupset.

Shimano used the same actuation ratio (ratio of cable length pulled to lateral movement of derailleur cage) for road 8, 9 and 10 speed systems then changed for 11 speed. Theoretically a 9 speed derailleur will work with a 10 speed shifter and cassette.

On the MTB side the change to a different actuation ratio was made at 10 speed, so a 9 speed MTB derailleur will not work with a 10 speed shifter and cassette.

A well known exception is road Tiagra 10 speed 4700 series which uses a road 11 speed actuation ratio (although I'm not sure that it's exactly the same, a Tiagra 4700 10 speed derailleur may not work with a 105 5800 or Ultegra 6800 group.)

Indexing is not too hard to get right (if you have matching shifters and derailleurs of course). Park Tool has a good page that explains how it works.

If you cannot get the indexing right at both ends of the cassette you may have another problem. A bent derailleur hanger is a common culprit. Cables hanging up in the housings is another.

If the chain is skipping over sprocket teeth rather than ghost shifting you likely have worn sprockets.