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My clematis in one section of the garden has yellow leaves with green veins which I believe is an iron deficiency. It is just the one clematis that is affected and I believe the spot it is in rain washes out nutrients over time. I have added compost, mulch and liquid seaweed. Last year I used a synthetic fix with a "Ironite Plus" spike. Is there an organic addition for this problem?

There is.
Being iron deficient does not necessarily mean that you need to add iron. There may be another nutrient lacking which stops the plant being able to take up the iron. If the area is leaching then chances are it is potassium you are lacking.
Eat a whole bunch of bananas, chop the skins up and dig them in. It should slowly recover.

Let me also add that clematis are incredibly hungry plants and a top dressing each year of manure is a good idea.

Very interesting, thanks Jess.

Thanks Jess, it looks like I'll be separating the banana peels from the rest of the compost for awhile. The manure is in the compost already but I can add more.

Can I just ask one thing NJ. Are all the leaves chlorotic or is it mainly the old growth or new growth?

Jess, The yellowing started at the lower, older leaves and worked their way up to the newer leaves. Some at the very top are still green. I wish it was easier to post photos so I could show you how it looks.

Ok, that still says potassium to me.
Here is a chart that shows deficiencies on a passiflora but it can be used for any climber.

https://www.passionflow.co.uk/mineral.htm

As far as manure goes add neat around the plant, not upto it but I normally add a good 2/3 inches each spring. Dig it in to the soil as it is a bit late. I use chicken manure pellets thruough the growing season but I believe you cannot use them in US?

Thanks again Jess.
I'm not sure about the pelleted chicken manure. Maybe someone knows?